Nanocomposites

ABSTRACT

This invention provides composite materials comprising nanostructures (e.g., nanowires, branched nanowires, nanotetrapods, nanocrystals, and nanoparticles). Methods and compositions for making such nanocomposites are also provided, as are articles comprising such composites. Waveguides and light concentrators comprising nanostructures (not necessarily as part of a nanocomposite) are additional features of the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/212,014, filed Sep. 17, 2008, which is a continuation of U.S. patentapplication Ser. No. 11/696,088, filed Apr. 3, 2007, which is acontinuation of U.S. patent application Ser. No. 11/342,087, filed Jan.26, 2006, now U.S. Pat. No. 7,228,050, which is a continuation of U.S.patent application Ser. No. 10/656,916, filed Sep. 4, 2003,“Nanocomposites” Mihai Buretea et al., now U.S. Pat. No. 7,068,898,which claims priority to and benefit of U.S. Provisional PatentApplication No. 60/408,722, filed Sep. 5, 2002, “Nanocomposites” MihaiBuretea et al. Each of these applications is incorporated herein byreference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention is in the field of nanocomposites. Moreparticularly, the invention includes composite materials comprisingnanostructures (e.g., nanowires, nanorods, branched nanowires,nanotetrapods, nanocrystals, quantum dots, and nanoparticles), methodsand compositions for making such composites, and articles comprisingsuch composites. Waveguides and light concentrators comprisingnanostructures that are not necessarily part of a nanocomposite are alsofeatures of the invention.

BACKGROUND OF THE INVENTION

A composite material (a composite) is formed by combining two or morematerials that have different properties. The composite typically hasproperties different from those of its constituent materials, but withinthe composite the original materials can still be identified (they donot dissolve; an interface is maintained between them). Typically, onematerial, called the matrix, surrounds and binds together discrete units(e.g., particles, fibers, or fragments) of a second material, called thefiller.

Many composite materials are currently known and widely used, forexample, concrete (a composite in which the matrix is cement and thefiller is aggregate), fiberglass (glass fibers in a plastic matrix), andmany other types of reinforced plastics. However, there is continueddemand for novel composites with desirable properties for manyapplications.

For example, the electronics industry utilizes materials that have highdielectric constants and that are also flexible, easy to process, andstrong. Finding single component materials possessing these propertiesis difficult. For example, high dielectric constant ceramic materialssuch as ferroelectric SrTiO₃, BaTiO₃, or CaTiO₃ are brittle and areprocessed at high temperatures that are incompatible with currentmicrocircuit manufacturing processes, while polymer materials are veryeasy to process but have low dielectric constants. Composite materialswith micron-scale ferroelectric ceramic particles as the filler inliquid crystal polymer, fluoropolymer, or thermoplastic polymer matricesare taught in U.S. Pat. No. 5,962,122 to Walpita et al (Oct. 5, 1999)entitled “Liquid crystalline polymer composites having high dielectricconstant,” U.S. Pat. No. 5,358,775 to Horn et al (Oct. 25, 1994)entitled “Fluoropolymeric electrical substrate material exhibiting lowthermal coefficient of dielectric constant,” U.S. Pat. No. 5,154,973 toImagawa et al (Oct. 13, 1992) entitled “Composite material fordielectric lens antennas,” and U.S. Pat. No. 4,335,180 to Traut (Jun.15, 1982) entitled “Microwave circuit boards.” However, these materialsdo not possess ideal processing characteristics. For example, they aredifficult to form into the thin uniform films used for manymicroelectronics applications.

Novel materials would also be useful in other industries, for example,in solar energy technology. The development of solar energy technologyis primarily concerned with reducing the cost of energy conversion. Thisis typically achieved in one of two ways: 1) increasing the conversionefficiency of light in a solar cell without proportionately increasingits cost, or 2) increasing the size of the cell without proportionatelyincreasing its cost. In the first case, the same number of photons hitthe solar cell, but a larger number of them are converted intoelectricity (or the ones that are converted are converted at a highertotal power). In the second, the conversion efficiency is the same, butthe larger surface area means that more photons are collected per unittime. Since the sun is free, this results in improved cost efficiency.Unfortunately, at the moment neither of these strategies is effective.The complexity of increased-efficiency solar cells causes their cost tobe substantially greater than the increase in performance. Similarly,larger solar panels are proportionately more expensive due todifficulties in fabricating uniform devices over large areas.

Among other aspects, the present invention provides high dielectricconstant nanocomposites that overcome the processing issues noted aboveand solar concentrators comprising nanostructures. A completeunderstanding of the invention will be obtained upon review of thefollowing.

SUMMARY OF THE INVENTION

The present invention provides nanocomposites (composite materialscomprising nanostructures such as nanowires, branched nanowires,nanotetrapods, nanocrystals, and nanoparticles, for example),compositions and methods for making such nanocomposites, and articlescomprising such composites.

One aspect of the invention provides waveguides and light concentratorscomprising nanostructures, which in some but not all embodiments areprovided as part of a nanocomposite. The nanostructures absorb lightimpinging on the waveguide or light concentrator and re-emit light. Thenanostructures can be located and/or oriented within the waveguide orlight concentrator in a manner that increases the percentage ofre-emitted light that can be waveguided. For example, the nanostructurescan be located and/or oriented within a light concentrator in such amanner that a greater percentage of the reemitted light is waveguided(and can thus be collected at the edge of the concentrator) than wouldbe waveguided if emission by the collection of nanostructures wereisotropic (equal in every direction).

One class of embodiments provides a waveguide comprising a cladding(e.g., a material that has a lower refractive index than the core, e.g.,a lower refractive index solid, liquid, or gas, e.g., air) and a core,where the core comprises one or more nanowires or branched nanowires(e.g., nanotetrapods) and a matrix. The first and second surfaces of thecore are substantially parallel so light emitted by the nanowires orbranched nanowires can be efficiently waveguided by total internalreflection, and the core has a higher index of refraction than thecladding, for a similar reason. The nanowires or branched nanowires cancomprise essentially any convenient material (e.g., a fluorescentmaterial, a semiconducting material) and can comprise essentially asingle material or can be heterostructures. The size of thenanostructures (e.g., the diameter and/or aspect ratio of nanowires) canbe varied. In embodiments in which the core comprises a plurality ofnanowires, the nanowires can be either randomly or substantiallynonrandomly oriented (e.g., with a majority of the nanowires being morenearly perpendicular than parallel to a surface of the core, or with thenanowires forming a liquid crystal phase). Nonrandom orientation of thenanowires can increase the efficiency of the waveguide by increasing thepercentage of light that is reemitted at angles greater than thecritical angle for the particular core-cladding combination. Thewaveguides can be connected to a collector for collecting waveguidedlight, and can be used in stacks to form a multilayer lightconcentrator, in which the different layers comprise waveguides that canbe optimized to collect light of different wavelengths.

Another class of embodiments provides a waveguide comprising a cladding(e.g., a material that has a lower refractive index than the core, e.g.,a lower refractive index solid, liquid, or gas, e.g., air), a firstcore, and a first layer that comprises one or more nanostructures. Thefirst layer is distributed on but is not necessarily in contact with thefirst core, whose first and second surfaces are substantially parallel.Some embodiments further comprise a second core. The first layer can bein direct contact with the first and/or second core(s), or can beseparated from either or both, e.g., by a layer of a material whoserefractive index is between that of the first layer and the core. Thefirst layer preferably has a thickness less than about one wavelength ofthe light emitted by the nanostructures. The nanostructures can benanowires, nanocrystals, or branched nanowires (e.g., nanotetrapods).The nanostructures can comprise essentially any convenient material(e.g., a fluorescent material, a semiconducting material) and cancomprise essentially a single material or can be heterostructures. Thesize of the nanostructures (e.g., the diameter and/or aspect ratio ofnanowires) can be varied. The nanostructures can be provided in variousmanners, e.g., as substantially pure nanostructures or as part of ananocomposite. In embodiments in which the waveguide comprises aplurality of nanowires, the nanowires can be either randomly orsubstantially nonrandomly oriented (e.g., with a majority of thenanowires being more nearly perpendicular than parallel to a surface ofthe first core, or with the nanowires forming a liquid crystal phase).Nonrandom orientation of the nanowires can increase the efficiency ofthe waveguide by increasing the percentage of light that is reemitted atangles greater than the critical angle. The waveguides can be connectedto a collector for collecting waveguided light, and can be used instacks to form a multilayer light concentrator, in which the differentlayers comprise waveguides that can be optimized to collect light ofdifferent wavelengths.

Another aspect of the invention provides various nanocomposites. Onecomposite material comprises a plurality of nanowires and a polymeric orsmall molecule or molecular matrix that is used to orient the nanowires.Another class of embodiments provides composites comprising one or morenanostructures (for example, nanowires, nanocrystals, or branchednanowires, e.g. nanotetrapods) and a polymeric matrix comprisingpolysiloxane (e.g., polydimethylsiloxane). The nanostructures cancomprise essentially any material (e.g., a ferroelectric, fluorescent,or semiconducting material). The composite can further comprise anadditive such as e.g. a surfactant or solvent. Articles comprising suchcomposites (e.g., an LED, laser, waveguide, or amplifier) are alsofeatures of the invention.

Yet another class of embodiments provides nanocomposites comprising asmall molecule or molecular matrix or a matrix comprising an organicpolymer or an inorganic glass and one or more branched nanowires (e.g.,nanotetrapods) or ferroelectric or semiconducting nanowires having anaspect ratio greater than about 10. The size of the nanostructures(e.g., the diameter and/or aspect ratio of nanowires) can be varied. Inembodiments in which the composite comprises a plurality of nanowires,the nanowires can be either randomly or substantially nonrandomlyoriented. For example, the composite can be formed into a thin film(strained or unstrained) within which a majority of the nanowires can besubstantially parallel to or more nearly perpendicular than parallel toa surface of the film.

An additional class of embodiments provides composite materialscomprising nanostructures and a polymeric matrix, a small molecule ormolecular matrix, or a glassy or crystalline inorganic matrix where thecomposite is distributed on a first layer of a material that conductssubstantially only electrons or substantially only holes. The compositeand the first layer can be in contact or can be separated, for example,by a second layer comprising a conductive material. The first layer canbe distributed on an electrode, and can be in contact with the electrodeor separated from it, for example, by a third layer comprising aconductive material. The conductive material may conduct electrons orholes or both.

In another class of embodiments, the invention provides nanocompositesthat support charge recombination or charge separation. These compositescomprise a matrix and one or more nanostructures (e.g., nanocrystals,nanowires, branched nanowires, or nanotetrapods), where semiconductingmaterials comprising the matrix and/or the nanostructures have a type Ior type II band offset with respect to each other.

An additional class of embodiments provides composites comprisingnanostructures and a polymeric or small molecule or molecular matrix, inwhich the components of the matrix have an affinity for the surface ofthe nanostructure or for surface ligands on the nanostructures. Forexample, the surface ligands can each comprise a molecule found in thesmall molecule or molecular matrix or a derivative thereof or a monomerfound in the polymeric matrix or a derivative thereof.

Another class of embodiments provides composite materials comprising oneor more ferroelectric nanowires or nanoparticles and a small molecule ormolecular matrix or a matrix comprising one or more polymers (e.g., anorganic, inorganic, or organometallic polymer). The nanowires ornanoparticles can comprise essentially any convenient ferroelectricmaterial, and their size (e.g., their diameter and/or aspect ratio) canbe varied. The dielectric constant of the composite can be adjusted byadjusting the amount of ferroelectric nanowires or nanoparticlesincluded in the composite. The composite (or its matrix) can furthercomprise an additive, for example, a surfactant, solvent, catalyst,plasticizer, antioxidant, or strengthening fiber. The composite materialcan be formed into a film or applied to a substrate. An additionalembodiment provides a composition comprising such a composite; thecomposition comprises particles of the composite material, at least onesolvent whose concentration can be varied, and at least one glue agent(e.g., a polymer or cross-linker). The composition can form a film,e.g., after application to a substrate.

Compositions that can be used to form a nanocomposite comprisingferroelectric nanowires or nanoparticles are another feature of theinvention. In one embodiment, the composition comprises one or moreferroelectric nanowires or nanoparticles, at least one solvent, and oneor more polymers. The polymers can be provided in any of a number offorms. For example, the polymer can be soluble in the solvent or cancomprise oligomers soluble in the solvent, or the polymer can compriseemulsion polymerized particles. The materials and size of the nanowiresand nanoparticles can be varied essentially as described above. Thecomposition can further comprise a glue agent, cross-linking agent,surfactant, or humectant. The consistency of the composition can becontrolled (e.g., by varying the solvent concentration) to make thecomposition suitable for use as an inkjet printing ink or screenprintingink, or for brushing or spraying onto a surface or substrate. Thecomposition can be used to form a film (e.g., a high dielectricnanocomposite film).

In a similar embodiment, the composition comprises one or moreferroelectric nanowires or nanoparticles, at least one solvent, and atleast one monomeric precursor of at least one polymer. The materials andsize of the nanowires and nanoparticles can be varied essentially asdescribed above. The composition can further comprise a catalyst,cross-linking agent, surfactant, or humectant. The consistency of thecomposition can be controlled (e.g., by varying the solventconcentration) to make the composition suitable for use as an inkjetprinting ink or screenprinting ink, or for brushing or spraying onto asurface or substrate. The composition can be used to form a film (e.g.,a high dielectric nanocomposite film).

Methods for making the composite materials and compositions describedabove provide an additional feature of the invention.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. The following definitionssupplement those in the art and are directed to the current applicationand are not to be imputed to any related or unrelated case, e.g., to anycommonly owned patent or application. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice for testing of the present invention, the preferred materialsand methods are described herein. Accordingly, the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting.

As used in this specification and the appended claims, the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “ananostructure” includes a plurality of such nanostructures, and thelike.

An “aspect ratio” is the length of a first axis of a nanostructuredivided by the average of the lengths of the second and third axes ofthe nanostructure, where the second and third axes are the two axeswhose lengths are most nearly equal each other. For example, the aspectratio for a perfect rod would be the length of its long axis divided bythe diameter of a cross-section perpendicular to (normal to) the longaxis.

A “branched nanowire” is a nanostructure having three or more arms,where each arm has the characteristics of a nanowire, or a nanostructurehaving two or more arms, each arm having the characteristics of ananowire and emanating from a central region that has a distinct crystalstructure, e.g., having cubic symmetry, e.g., where the angle betweenany two arms is approximately 109.5 degrees. Examples include, but arenot limited to, bipods, tripods, and nanotetrapods (tetrapods). Abranched nanowire can be substantially homogenous in material propertiesor can be heterogeneous (a heterostructure). For example, a branchednanowire can comprise one material at the center of the branch which isa single crystal structure and a second material along the arms of thestructure that is a second crystal structure, or the materials alongeach of the arms can differ, or the material along any single arm canchange as a function of length or radius of the arm. Branched nanowirescan be fabricated from essentially any convenient material or materials.Branched nanowires can comprise “pure” materials, substantially purematerials, doped materials and the like, and can include insulators,conductors, and semiconductors.

The terms “crystalline” or “substantially crystalline,” when used withrespect to nanostructures, refer to the fact that the nanostructurestypically exhibit long-range ordering across one or more dimensions ofthe structure. It will be understood by one of skill in the art that theterm “long range ordering” will depend on the absolute size of thespecific nanostructures, as ordering for a single crystal cannot extendbeyond the boundaries of the crystal. In this case, “long-rangeordering” will mean substantial order across at least the majority ofthe dimension of the nanostructure. In some instances, a nanostructurecan bear an oxide or other coating, or can be comprised of a core and atleast one shell. In such instances it will be appreciated that theoxide, shell(s), or other coating need not exhibit such ordering (e.g.it can be amorphous, polycrystalline, or otherwise). In such instances,the phrase “crystalline,” “substantially crystalline,” “substantiallymonocrystalline,” or “monocrystalline” refers to the central core of thenanostructure (excluding the coating layers or shells). The terms“crystalline” or “substantially crystalline” as used herein are intendedto also encompass structures comprising various defects, stackingfaults, atomic substitutions, and the like, as long as the structureexhibits substantial long range ordering (e.g., order over at leastabout 80% of the length of at least one axis of the nanostructure or itscore). In addition, it will be appreciated that the interface between acore and the outside of a nanostructure or between a core and anadjacent shell or between a shell and a second adjacent shell maycontain non-crystalline regions and may even be amorphous. This does notprevent the nanostructure from being crystalline or substantiallycrystalline as defined herein.

The “diameter of a nanocrystal” refers to the diameter of across-section normal to a first axis of the nanocrystal, where the firstaxis has the greatest difference in length with respect to the secondand third axes (the second and third axes are the two axes whose lengthsmost nearly equal each other). The first axis is not necessarily thelongest axis of the nanocrystal; e.g., for a disk-shaped nanocrystal,the cross-section would be a substantially circular cross-section normalto the short longitudinal axis of the disk. Where the cross-section isnot circular, the diameter is the average of the major and minor axes ofthat cross-section.

The “diameter of a nanowire” refers to the diameter of a cross-sectionnormal to the major principle axis (the long axis) of the nanowire.Where the cross-section is not circular, the diameter is the average ofthe major and minor axes of that cross-section.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In certainembodiments, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanowire, or the centerof a nanocrystal, for example. (A shell need not completely cover theadjacent materials to be considered a shell or for the nanostructure tobe considered a heterostructure; for example, a nanocrystalcharacterized by a core of one material covered with small islands of asecond material is a heterostructure.) In other embodiments, thedifferent material types are distributed at different locations withinthe nanostructure; e.g., along the major (long) axis of a nanowire oralong a long axis of arm of a branched nanowire. Different regionswithin a heterostructure can comprise entirely different materials, orthe different regions can comprise a base material (e.g., silicon)having different dopants or different concentrations of the same dopant.

The term “monocrystalline” when used with respect to a nanostructureindicates that the nanostructure is substantially crystalline andcomprises substantially a single crystal. When used with respect to ananostructure heterostructure comprising a core and one or more shells,“monocrystalline” indicates that the core is substantially crystallineand comprises substantially a single crystal.

A “nanocrystal” is a nanostructure that is substantiallymonocrystalline. Nanocrystals typically have an aspect ratio betweenabout 0.1 and about 1.5 (e.g., between about 0.1 and about 0.5, betweenabout 0.5 and about 1, or between about 1 and about 1.5). Thus,nanocrystals include, for example, substantially spherical nanocrystalswith aspect ratios between about 0.8 and about 1.2 and disk-shapednanocrystals. Nanocrystals typically have a diameter between about 1.5nm and about 15 nm (e.g., between about 2 nm and about 5 nm, betweenabout 5 nm and about 10 nm, or between about 10 nm and about 15 nm).Nanocrystals can be substantially homogeneous in material properties, orin certain embodiments can be heterogeneous (e.g. heterostructures). Inthe case of nanocrystal heterostructures comprising a core and one ormore shells, the core of the nanocrystal is substantiallymonocrystalline, but the shell(s) need not be. The nanocrystals can befabricated from essentially any convenient material or materials. Thenanocrystals can comprise “pure” materials, substantially purematerials, doped materials and the like, and can include insulators,conductors, and semiconductors.

A “nanoparticle” is any nanostructure having an aspect ratio less thanabout 1.5. Nanoparticles can be of any shape, and include, for example,nanocrystals, substantially spherical particles (having an aspect ratioof about 0.9 to about 1.2), and irregularly shaped particles.Nanoparticles can be amorphous, crystalline, partially crystalline,polycrystalline, or otherwise. Nanoparticles can be substantiallyhomogeneous in material properties, or in certain embodiments can beheterogeneous (e.g. heterostructures). The nanoparticles can befabricated from essentially any convenient material or materials. Thenanoparticles can comprise “pure” materials, substantially purematerials, doped materials and the like, and can include insulators,conductors, and semiconductors.

A “nanostructure” is a structure having at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanowires, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, and the like.Nanostructures can be substantially homogeneous in material properties,or in certain embodiments can be heterogeneous (e.g. heterostructures).The nanostructures can be fabricated from essentially any convenientmaterial or materials. The nanostructures can comprise “pure” materials,substantially pure materials, doped materials and the like, and caninclude insulators, conductors, and semiconductors. A nanostructure canoptionally comprise one or more surface ligands (e.g., surfactants).

A “nanotetrapod” is a generally tetrahedral branched nanowire havingfour arms emanating from a central region, where the angle between anytwo arms is approximately 109.5 degrees.

A “nanowire” is a nanostructure that has one principle axis that islonger than the other two principle axes. Consequently, the nanowire hasan aspect ratio greater than one; nanowires of this invention have anaspect ratio greater than about 1.5 or greater than about 2. Shortnanowires, sometimes referred to as nanorods, typically have an aspectratio between about 1.5 and about 10. Longer nanowires have an aspectratio greater than about 10, greater than about 20, greater than about50, or greater than about 100, or even greater than about 10,000. Thediameter of a nanowire is typically less than about 500 nm, preferablyless than about 200 nm, more preferably less than about 150 nm, and mostpreferably less than about 100 nm, about 50 nm, or about 25 nm, or evenless than about 10 nm or about 5 nm. The nanowires of this invention canbe substantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g. nanowire heterostructures). Thenanowires can be fabricated from essentially any convenient material ormaterials. The nanowires can comprise “pure” materials, substantiallypure materials, doped materials and the like, and can includeinsulators, conductors, and semiconductors. Nanowires are typicallysubstantially crystalline and/or substantially monocrystalline, but canbe, e.g., polycrystalline or amorphous. Nanowires can have a variablediameter or can have a substantially uniform diameter, that is, adiameter that shows a variance less than about 20% (e.g., less thanabout 10%, less than about 5%, or less than about 1%) over the region ofgreatest variability and over a linear dimension of at least 5 nm (e.g.,at least 10 nm, at least 20 nm, or at least 50 nm). Typically thediameter is evaluated away from the ends of the nanowire (e.g. over thecentral 20%, 40%, 50%, or 80% of the nanowire). A nanowire can bestraight or can be e.g. curved or bent, over the entire length of itslong axis or a portion thereof. In certain embodiments, a nanowire or aportion thereof can exhibit two- or three-dimensional quantumconfinement. Nanowires according to this invention can expressly excludecarbon nanotubes, and, in certain embodiments, exclude “whiskers” or“nanowhiskers”, particularly whiskers having a diameter greater than 100nm, or greater than about 200 nm.

The phrase “substantially nonrandom” used to describe the orientation ofnanowires means that the nanowires do not occupy a purely randomdistribution of orientations with respect to each other. A collection ofnanowires is substantially nonrandomly oriented if, when the position ofeach nanowire is represented as a vector of unit length in athree-dimensional rectangular coordinate system, at least one componentof the vector average of the nanowires' orientations is non-zero (whenrepresenting a nanowire by a vector, any intrinsic difference betweenthe two ends of the nanowire can typically be ignored). For example, thenanowires in a collection of nanowires (e.g., the nanowires in acomposite material comprising nanowires) would have substantiallynonrandom orientations if a higher percentage of the nanowires pointedin one direction (or in one of at least two specific directions) than inany other direction (e.g., if at least 10%, at least 50%, at least 75%,or at least 90% of the nanowires pointed in a particular direction). Asanother example, nanowires in a thin film of a composite comprisingnanowires would be substantially nonrandomly oriented if a majority ofthe nanowires had their long axes more nearly perpendicular thanparallel to a surface of the film (or vice versa) (the nanowires can besubstantially nonrandomly oriented yet not point in at least onespecific direction). The preceding examples are for illustration only; acollection of nanowires could possess less order than these examples yetstill be substantially nonrandomly oriented.

A “surface ligand” of a nanostructure is a molecule that has an affinityfor and is capable of binding to at least a portion of thenanostructure's surface. Examples include various surfactants. Surfaceligands or surfactants can comprise e.g. an amine, a phosphine, aphosphine oxide, a phosphonate, a phosphonite, a phosphinic acid, aphosphonic acid, a thiol, an alcohol, an amine oxide, a polymer, amonomer, an oligomer, or a siloxane.

A “type I band offset” between two semiconducting materials means thatboth the conduction band and the valence band of the semiconductor withthe smaller bandgap are within the bandgap of the other semiconductor.

A “type II band offset” between two semiconducting materials means thateither the conduction band or the valence band, but not both, of onesemiconductor is within the bandgap of the other semiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a quantum dot solar concentrator.

FIG. 2 schematically illustrates a waveguide in which the core comprisessubstantially nonrandomly oriented nanowires.

FIG. 3 schematically depicts a waveguide comprising a first core and afirst layer in contact with each other.

FIG. 4 schematically depicts a waveguide comprising a first layer incontact with two cores.

FIG. 5 schematically depicts a waveguide comprising a first, a second,and a third layer and two cores.

FIG. 6 illustrates absorption and emission by the organic dyefluorescein (Panel A) and by spherical CdSe nanocrystals with an aspectratio of about 1.1 and an average diameter of about 2.3 nm (Panel B).

FIG. 7 depicts examples of vinyl-terminated polydimethylsiloxaneoligomers (Panel A) and siloxane cross-linkers (Panel B) and illustratesthe formation of cross-links between them (Panel C).

FIG. 8 schematically depicts examples of type II (Panels A and C) andtype I (Panels B and D) band offsets.

DETAILED DESCRIPTION

Composite materials comprising nanostructures (nanocomposites) areprovided, along with articles comprising nanocomposites and methods andcompositions for making such composites.

One class of embodiments provides waveguides and light concentratorscomprising nanostructures. This class of embodiments is based on theability of certain nanostructures (e.g., nanostructures comprising afluorescent material) to absorb and re-emit light that can then bewaveguided by total internal reflection within at least one core. Inmany but not all embodiments, the nanostructures are provided in theform of a nanocomposite. In some embodiments, the nanostructures aresubstantially nonrandomly oriented to increase the efficiency of thewaveguide or light concentrator.

Methods for orienting nanostructures within a composite material arediscussed, including using the matrix or components thereof to orientthe nanostructures. Accordingly, one aspect of the invention providesnanocomposites in which the matrix is used to orient nanowires.

The invention also provides nanocomposites comprising nanostructures anda polysiloxane matrix, as well as articles comprising such composites.Another class of embodiments provides nanocomposites comprising branchednanowires or ferroelectric or semiconducting nanowires. Yet anotherclass of embodiments includes the use of nanocomposites with blockinglayers that conduct substantially only electrons or substantially onlyholes. Other nanocomposites are provided that support chargerecombination or charge separation (e.g., for use in luminescent orphotovoltaic devices). An additional class of embodiments providescomposites in which interaction between the nanostructures and thematrix is enhanced, for example, by surface ligands on thenanostructures.

One general class of embodiments provides nanocomposites that compriseferroelectric nanowires or ferroelectric nanoparticles and that can thuspossess high dielectric constants. Compositions for making suchnanocomposites (e.g., compositions suitable for use as inkjet or screenprinting inks) are also provided, as are methods for making all theabove composites and compositions. The following sections describe theinvention in more detail.

Waveguides and Light Concentrators

One aspect of the present invention provides waveguides and lightconcentrators (e.g., solar concentrators) comprising nanostructures.Energy (e.g., light) is absorbed and re-emitted by the nanostructuresand is waveguided by total internal reflection within a core. In manyembodiments, the nanostructures are provided in the form of ananocomposite.

Dye and Quantum Dot Solar Concentrators

As mentioned previously, typical approaches to reducing the cost ofsolar energy conversion are to increase the efficiency and/or size of asolar cell without proportionately increasing its cost. A differentmethod of improving cost performance is to increase the intensity oflight on a single cell. Assuming that the cell does not burn or saturateunder the increased illumination, and assuming that the apparatus usedto concentrate the light does not cost proportionately more than theincrease in intensity, this can also produce an improvement in costefficiency. One idea for this type of device is to take a large plasticor glass sheet and dope it with organic dye molecules that absorb andre-emit light with high efficiency. Solar concentrators of this type aredescribed in e.g., Weber et al. (1976) App. Opt. 15, 2299 andGoetzberger et al. (1977) Appl. Phys. 14, 123. As illustrated in FIG. 1,light (4, squiggly arrows) impinging on the sheet 1 from a light source3 located above the sheet is absorbed by the dye molecules (5, circles)and then re-emitted in all directions equally (isotropic emission).(Strictly speaking, most dye molecules emit light that is polarizedalong a single axis. In this type of solar concentrator, the individualdye molecules are isotropically oriented relative to each other withinthe sheet. As a result, the average emission profile of all of the dyemolecules looks isotropic.) A few emitted light rays are indicated bysolid arrows; continuing paths for some of these emitted rays areindicated by dashed arrows. Light that is re-emitted at angles that aregreater than the critical angle for the interface between the sheet andthe surrounding air is waveguided by total internal reflection andtravels to the edge of the sheet before it escapes. Thus, light emittedat angles greater than the critical angle can be collected at the edgeby a solar cell 2. The critical angle (Θ_(crit)) for a given sheet-airinterface depends on the indices of refraction of the sheet and air:Θ_(crit)=sin⁻¹(n_(r)/n_(i)), where n_(r) is the refractive index of airand n_(i) is the refractive index of the sheet. As illustrated for thecritical angle 6, an angle of incidence is measured between the incidentray 7 and a line 8 normal to the surface of the sheet.

By using a very large area concentrator, the intensity of the impinginglight can be greatly concentrated, as represented by the equation C=S*G,where C is the ratio of light concentration collected, S is the ratio ofthe surface area of the sheet being illuminated by impinging light tothe area of the collector at the edge of the sheet, and G representslosses in the concentrator that affect the ratio of the photons strikingthe surface of the concentrator to the number of photons that get guidedto the edge of the sheet. In a standard concentrator, there are a numberof inefficiencies that influence G. First, dye molecules only absorb anarrow band of wavelengths, and so most of the light that hits theconcentrator is not absorbed, but simply passes through and is lost.This can be a substantial loss in overall efficiency. Second, dyemolecules emit with finite quantum efficiency. As a result, even thephotons that are absorbed do not all get re-emitted. Third, dyemolecules are photo-unstable and eventually the concentrator stopsworking as the dye photobleaches. Finally, of the photons that arere-emitted, only those that emit at angles greater than the criticalangle actually get wave guided and eventually collected. All otherphotons are lost. Other factors such as losses due to reabsorption orscatter within the concentrator also affect G, however, the factorsabove represent the major contributors to loss in this type ofconcentrator. As a result of these issues, solar concentrators of thistype have not been implemented in a commercial product to date.

In order to improve the overall efficiency of a concentrator of the typedescribed above, improvements in the following five characteristics arepossible: 1) absorption efficiency; 2) absorption bandwidth (the breadthof the absorption spectrum); 3) quantum yield of the fluorophores in theconcentrator; 4) photostability of the fluorophores in the concentrator;and 5) the angular distribution of intensities emitted from thefluorophores in the concentrator after illumination from above (thepercentage of the re-emitted photons have an angle greater than thecritical angle and therefore get waveguided).

An improvement upon the dye molecule concentrator was made by replacingthe dye molecules with quantum dots. See, e.g., Barnham et al. (2000)“Quantum-dot concentrator and thermodynamic model for the globalred-shift” Applied Physics Letters 76, 1197-1199. Quantum dots have anumber of substantial advantages over dye molecules: 1) they areextremely photostable and do not bleach, even under solar radiation; 2)they have an extremely broad absorption spectrum with extinctioncoefficients as much as 10 times greater than typical organic dyemolecules and therefore absorb solar radiation much more efficientlythan dye molecules; and 3) they can be fabricated with quantumefficiencies as high as 80%. Use of quantum dots in a solar concentratortherefore improves performance of the concentrator by improving thefirst four contributions to G described above (absorption efficiency,absorption bandwidth, quantum yield, and photostability). Like acollection of dye molecules, however, the collection of quantum dotsemits isotropically and thus their use does not increase the percentageof emitted photons that are waveguided (the fifth contribution to G). Inthe case of solar concentrators comprising quantum dots, there are twoeffects that produce an isotropic emission profile from the collectionof dots. First, for some quantum dots with a wurtzite crystal structure,the light emitted is not strictly polarized in the traditional sense,but along a 2-dimensional dipole moment oriented in the x-y plane of thenanocrystal. In other nanocrystals with a more symmetric crystalstructure such as zincblend, light is emitted isotropically from thecrystal in all three dimensions. Second, the quantum dots are notoriented within the sheet and therefore, even if they were polarized,they would still have an ensemble average emission profile that isisotropic. In both existing dye and quantum dot solar concentrators, itis significant that there is no consistent average orientation of theemission transition dipole to enhance collection efficiency as isdescribed in the present invention.

In a quantum dot concentrator having quantum dots embedded in atransparent sheet surrounded by air, where the collection of dots emitslight isotropically, at most ½*(cos(a sin(n₁/n₂))−cos(pi−a sin(n₁/n₂)))of the light emitted by the quantum dots can be waveguided and thuscollected, where n₁ is the refractive index of air and n₂ is therefractive index of the sheet, cos is cosine, a sin is arcsine, and piis the Greek letter approximately equal to 3.14159265.

Ideas for improving the performance of a quantum dot concentratorinclude incorporating quantum dots with higher fluorescence quantumefficiency (e.g., greater than 30% or greater than 50%) or quantum dotswith a substantially monodisperse size and/or shape distribution (seee.g., US patent application 20020071952 by Bawendi et al entitled“Preparation of nanocrystallites”).

Light Concentrators

In a first general class of embodiments, the light concentrators of thisinvention comprise at least one core, at least one adjacent materialthat has a lower index of refraction than the core and that is incontact with at least a portion of a surface of the core, and aplurality of nanostructures. The nanostructures absorb light thatimpinges on a surface of the concentrator and re-emit light. Thelocation of the nanostructures within the concentrator and/or theorientation of the nanostructures is controlled such that the fractionof the light emitted by the nanostructures that is waveguided by thecore or cores is greater than ½*(cos(a sin(n₁/n₂))−cos(pi−asin(n₁/n₂))), where n₁ is the refractive index of the adjacent materialand n₂ is the refractive index of the core. (This fraction representsthe amount of light that would be emitted at angles greater than thecritical angle for a particular dielectric interface if the populationof emitters were located within the core and were collectively emittingisotropically.) Preferably at least 1%, more preferably at least 10%, ormost preferably at least 50% of the total nanostructures in theconcentrator are located or oriented such that greater than ½*(cos(asin(n₁/n₂))−cos(pi−a sin(n₁/n₂))), where n₁ is the refractive index ofthe adjacent material and n₂ is the refractive index of the core, of thelight emitted by the nanostructures is waveguided by the core(s). Atleast one collector for collecting the waveguided light is operablyconnected to the core, e.g., to an edge of the core. Any type ofcollector can be used for collecting the light and/or measuring itsintensity, for example, a detector, fiber optic cable, photocell, orsolar cell. Optionally, any edges or portions of an edge of the core notoccupied by the collector can be mirrored or silvered, so the waveguidedlight does not escape through these regions and decrease the efficiencyof the concentrator. The adjacent material can be e.g., a cladding, afirst layer comprising the nanostructures, or a layer of any lowrefractive index material.

The location of the nanostructures within the concentrator can becontrolled. For example, the nanostructures can be located within thecore or can be outside the core, e.g., in the material adjacent to thecore. Alternatively or in addition, the orientation of thenanostructures can be controlled. For example, the light concentratorcan comprise any nanostructures that have a definable unique axis ofsymmetry (e.g. a unique crystal axis such as the c-axis of a wurtzitenanocrystal, the elongated axis of a nanowire, or the long axis alongthe arm of a nanotetrapod). Placement of such nanostructures with asubstantially non-random distribution of orientations of the unique axise.g., relative to a surface of the concentrator or each other, canincrease the amount of light collected by the concentrator. For example,a vector average of the orientation of the nanostructures' unique axiscan have a nonzero component perpendicular to a surface of theconcentrator. As one example, a plurality of substantially sphericalnanocrystals (having an aspect ratio between about 0.8 and about 1.2)that have a wurtzite crystal structure can have a substantiallynon-random distribution of the c-axis of each wurtzite crystal and thusincrease the percentage of light that is reemitted at angles greaterthan the critical angle, e.g. for a particular core-claddingcombination.

In one embodiment, the light concentrator comprises at least one core, acladding having a lower index of refraction than the core, and aplurality of nanowires that are located inside the core. The nanowiresinside the core are substantially nonrandomly oriented in such a mannerthat, if a three-dimensional rectangular coordinate system is imposed onthe core and the position of each nanowire is represented by a vector ofunit length, the vector average of the nanowires' orientations has anonzero component perpendicular to a surface of the core. This resultsin greater than ½*(cos(a sin(n₁/n₂))−cos(pi−a sin(n₁/n₂))), where n₁ isthe refractive index of the cladding and n₂ is the refractive index ofthe core, of any light re-emitted by the nanowires being waveguided bythe core, as the following description will make clear.

Nanowires absorb light isotropically, but, unlike quantum dots, theyre-emit light in a non-isotropic pattern defined by a radiating dipoleoriented along the long axis of the nanowire. As a result, most of thefluorescence for a given nanowire is emitted in a directionperpendicular to the long axis of the nanowire. This is different thanthe case for fluorescent dye molecules and for quantum dots, which isone of the reasons why the present invention is so unique. In the caseof dye molecules, while emission from a single molecule isnon-isotropic, defined by a radiating dipole, the absorption is alsonon-isotropic, with an absorption dipole typically orientedsubstantially parallel (but not always exactly parallel) to the emissiondipole. Quantum dots, on the other hand, have isotropic absorption andemission (or isotropic absorption and 2-dimensional emission). In bothof these cases, there is very inefficient light collection by theconcentrator. For dye molecules, the molecules that absorb light arethose whose excitation dipole are oriented more parallel to thewaveguide surface than perpendicular. These molecules, which typicallyhave an emission dipole that is oriented in substantially the samedirection as the excitation dipole, will then re-emit more strongly in adirection perpendicular to the waveguide than parallel. As a result, thelight is not efficiently waveguided. In the case of quantum dots, allquantum dots absorb light, but there is no orientation that directslight efficiently parallel to the waveguide (at best, one of the twodipoles in the 2D transition dipole will be oriented to emit within theplane while the other emits out of the plane. As a result, efficientcollection is not achieved.

Because light emission by a single nanowire is non-isotropic, if thenanowires in a collection of nanowires (e.g., in a core) are randomlyoriented, emission by the collection of nanowires is isotropic, but ifthe nanowires are nonrandomly oriented, emission by the collection isnon-isotropic. In the example of nanowires in a waveguide core, thegreater the percentage of nanowires that have a component of their longaxis oriented perpendicular to the surface of the core is, or thegreater the component of the vector average of the nanowires'orientations perpendicular to the surface is, the greater the percentageof light reemitted at angles greater than the critical angle and thuswaveguided by the core is.

Thus, preferably at least 1%, more preferably at least 10%, or mostpreferably at least 50% of all the nanowires within the core aresubstantially nonrandomly oriented, in a direction perpendicular to thesurface of the core. Optionally, at least one collector for collectingthe waveguided light (e.g., a detector, fiber optic cable, photocell, orsolar cell) is operably connected to the core, e.g., to at least oneedge of the core.

In another embodiment, the light concentrator comprises at least onecore and a first layer comprising one or more nanostructures disposed ona surface of the core. The nanostructures absorb light that impinges ona surface of the concentrator and re-emit light, and greater than½*(cos(a sin(n₁/n₂))−cos(pi−a sin(n₁/n₂))), where n₁ is the refractiveindex of the first layer and n₂ is the refractive index of the core, ofthe re-emitted light is waveguided by the core(s).

As described in e.g. Lukosz (1981) “Light emission by multipole sourcesin thin layers. I. Radiation patterns of electric and magnetic dipoles”J. Opt. Soc. Am. 71,744-754; Lukosz et al. (1977) “Light emission bymagnetic and electric dipoles close to a plane dielectric interface. II.Radiation patterns of perpendicular oriented dipoles” J. Opt. Soc. Am.67, 1615-1619; Lukosz (1979) “Light emission by magnetic and electricdipoles close to a plane dielectric interface. III. Radiation patternsof dipoles with arbitrary orientation” J. Opt. Soc. Am. 69, 1495-1503;Fattinger et al. (1984) “Optical-environment-dependent lifetimes andradiation patterns of luminescent centers in very thin films” Journal ofLuminescence 31 & 32, 933-935; Lukosz and Kunz (1977) “Light emission bymagnetic and electric dipoles close to a plane interface. I. “Totalradiated power” J. Opt. Soc. Am. 67, 1607-1614; Lukosz and Kunz (1977)“Fluorescence lifetime of magnetic and electric dipoles near adielectric interface” Optics Communications 20:195-199; Chance et al.(1975) “Luminescent lifetimes near multiple interfaces: A quantitativecomparison of theory and experiment” Chem. Phys. Lett. 33:590-592;Lukosz and Kunz (1980) “Changes in fluorescence lifetimes induced byvariable optical environments” Phys. Rev. B 21:4814-4828; Chance et al.(1974) “Lifetime of an emitting molecule near a partially reflectingsurface” J. Chem. Phys. 60:2744-2748; Drexhage (1970) “Influence of adielectric interface on fluorescence decay time” J. Lumin. 1,2:693-701;and Chance et al. (1974) “Lifetime of an excited molecule near a metalmirror: Energy transfer in the Eu³⁺/silver system” J. Chem. Phys.60:2184-2185, a radiating dipole located on or near a dielectricinterface (an interface between two materials having differentrefractive indices) emits most of its light into the higher indexmaterial. Of particular importance, the light is emitted into the higherindex material at angles greater than the critical angle for thatinterface. Additionally, if the dipole is oriented at an anglesubstantially normal to the dielectric interface, an even greaterpercentage of the light is emitted beyond the critical angle.

Thus, in a preferred embodiment, the light concentrator comprises afirst layer comprising a plurality of nanowires disposed on a surface ofthe core and substantially nonrandomly oriented such that a vectoraverage of the nanowires' orientations has a nonzero componentperpendicular to the surface of the core. Preferably at least 1%, morepreferably at least 10%, and most preferably at least 50% of all thenanowires on the core are substantially nonrandomly oriented in thismanner, since the greater the degree of orientation perpendicular to thesurface of the core, the greater the efficiency of capture of there-emitted light.

In another embodiment, the nanowires need not be oriented (the nanowiresare randomly oriented). In still other embodiments, the first layer cancomprise nanostructures other than nanowires, for example, branchednanowires (e.g., nanotetrapods, tripods, or bipods) or nanocrystals(e.g., quantum dots). These nanostructures can be randomly ornonrandomly oriented. Optionally, at least one collector for collectingthe waveguided light (e.g., a detector, fiber optic cable, photocell, orsolar cell) is operably connected to the core, e.g., to at least oneedge of the core.

In all of the above embodiments, the light concentrators or layersthereof (e.g., the core(s), the first layer) can be e.g. either twodimensional or one dimensional. In the example of a two dimensionalcore, the core is substantially larger than necessary to support asingle optical mode at the wavelength that is re-emitted by thenanostructures in the concentrator in more than one dimension. In theexample of a one dimensional core, the core can support only a singlemode in two of three dimensions. In the one dimensional case, any otherlayers in the concentrator can be e.g. placed on a single surface ore.g. the layers can be created concentrically around a central layer (asis the case for an optical fiber). In this example, the central layercan be either the core or e.g. an adjacent material, a cladding, or afirst layer. Remaining layers are then built from the inside out.

Nanowires in the Core

One embodiment provides a waveguide that comprises a cladding and acore. The core comprises one or more nanowires, or one or more branchednanowires, and a matrix (the core comprises a nanocomposite). The firstand second surfaces of the core are substantially parallel to eachother, permitting the core to act as an efficient waveguide. The corehas a higher index of refraction (refractive index) than the cladding,and is in contact with the cladding over at least a majority of itsfirst and second surfaces.

The cladding comprises a material that has a lower refractive index thanthe core, e.g., a lower refractive index solid, liquid, or gas, e.g.,air, a low index plastic or polymer film or sheet, or any otherconvenient material having a smaller index of refraction than the core.It will be noted that the cladding need not be a single substance; thecladding in contact with the first surface of the core can comprise adifferent material than the cladding in contact with the second surface,for example.

As stated above, the refractive index of the core must be greater thanthat of the cladding. The larger the index difference at a dielectricinterface, the smaller the critical angle for that interface and thelarger the number of re-emitted photons that can be waveguided by totalinternal reflection and collected. This means the use of high indexcores is advantageous. For example, the core can have an index ofrefraction greater than about 1.35, greater than about 2.5, greater thanabout 3.3, or even greater than 4. Preferably, the core has an index ofrefraction between about 1.35 and about 4.

To optimize the number of photons that can be waveguided and collected,the matrix of the core is preferentially substantially nonabsorbing andnonscattering with respect to light at wavelengths absorbed and emittedby the nanowires or branched nanowires in the core. For a waveguide foruse in a solar concentrator, for example, the matrix is preferablysubstantially nonabsorbing and nonscattering to light in the visible,near-infrared, and infrared range, and thus is preferably substantiallynonabsorbing and nonscattering with respect to wavelengths greater thanabout 300 nm (e.g., greater than about 300 or 400 nm and less than about10 or 20 micrometers). In some instances, a physical property of thematrix such as e.g. its mechanical strength can be another criterionused to select an appropriate matrix material, since in some embodimentsthe waveguide (and typically also the core) is a flat sheet, which canbe of considerable size. (As will be evident, weaker materials can stillbe formed into large sheets, e.g. with the provision of suitablephysical support.)

The matrix can comprise e.g. a glass, a polymer, an organic polymer, aninorganic polymer, an organometallic polymer, a small molecule ormolecular matrix, a gel, a liquid, a crystal, a polycrystal, or amesoporous matrix; a number of these are known (e.g., the examplepolymers, glasses, and small molecules mentioned and/or referencedherein). Example materials include but are not limited to acrylic andpoly(methyl methacrylate); a large number of other suitable materialsare known in the art.

Either nanowires or branched nanowires (or a mixture thereof) can beused in the core. The branched nanowires can be nanotetrapods, oralternatively can be other branched structures (e.g., tripods orbipods). As described below, the diameter of the nanowires can bevaried, for example, to control the wavelengths emitted by thenanowires. In certain embodiments, the waveguides comprise nanowireshaving an average diameter between about 2 nm and about 100 nm (e.g.,between about 2 nm and about 50 nm, or between about 2 nm and about 20nm). Nanowires with shorter aspect ratios are typically preferred, so insome embodiments the nanowires have an aspect ratio between about 1.5and about 100, e.g. between about 5 and about 30.

In some embodiments, the waveguide core comprises a plurality ofnanowires, which can be either randomly or substantially nonrandomlyoriented. In a preferred embodiment, the orientation of the nanowires issubstantially nonrandom, with the vector average of the nanowires'orientations having a nonzero component perpendicular to the firstsurface of the core. Preferably at least 1%, more preferably at least10%, and most preferably at least 50% of all the nanowires in the coreare substantially nonrandomly oriented in this manner, since the greaterthe degree of orientation perpendicular to the surface of the core, thegreater the efficiency of capture of the re-emitted light. In oneembodiment, a majority of the nanowires each has its long axis orientedmore nearly perpendicular to than parallel to a surface of the core. Inone specific embodiment, the nanowires within the core form a liquidcrystal phase, in which each nanowire has its long axis orientedsubstantially perpendicular to a surface of the core. The use of such aliquid crystal phase results in a material that absorbs light fromsubstantially all directions strongly but emits light primarily atangles inside the plane of the array.

FIG. 2 illustrates an example of a waveguide in which the core 10comprises substantially nonrandomly oriented nanowires (9, cylinders).The nanowires absorb light (14, squiggly arrows) impinging on thewaveguide from a light source 13 and re-emit light. A few rays ofre-emitted light are indicated by solid arrows; continuing paths forsome of these emitted rays are indicated by dashed arrows. Light that isre-emitted at angles that are greater than the critical angle 6 for theinterface between the core and the cladding 12 (air in this example) iswaveguided by total internal reflection and travels to the edge of thecore before it escapes. Thus, light emitted at angles greater than thecritical angle can be collected at the edge by a collector 11. Thecritical angle (Θ_(crit)) for a given core-cladding interface depends onthe indices of refraction of the core and the cladding:Θ_(crit)=sin⁻¹(n_(r)/n_(i)), where n_(r) is the refractive index of thecladding and n_(i) is the refractive index of the core. Use of this typeof waveguide in a solar concentrator results in a concentratorbenefiting from all the improvements of a quantum dot solar concentratorbut also possessing greater efficiency, since a greater percentage ofthe re-emitted light is emitted at angles greater than the criticalangle for total internal reflection than in a quantum dot concentrator(in the nanowire concentrator, performance is improved by improving allfive contributions to G).

In a preferred embodiment, the waveguide core comprises a plurality ofnanowires that absorb light impinging on a surface of the core and emitlight. The nanowires are oriented in the core in such a manner that amajority of the emitted light is emitted an angle greater than thecritical angle for the specific core-cladding combination. A majority ofthe emitted light is thus waveguided and directed toward at least oneedge of the core. This type of waveguide can be an efficient lightconcentrator, since the waveguide can absorb light over a large surfacearea and direct it to an edge which has a much smaller area.

The one or more nanowires or branched nanowires can be fabricated fromessentially any convenient material. For example, the nanowires orbranched nanowires can comprise a semiconducting material, for example amaterial comprising a first element selected from group 2 or from group12 of the periodic table and a second element selected from group 16(e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe,MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and likematerials); a material comprising a first element selected from group 13and a second element selected from group 15 (e.g., GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, and like materials); a material comprising a group14 element (Ge, Si, and like materials); a material such as PbS, PbSe,PbTe, AlS, AlP, and AlSb; or an alloy or a mixture thereof. Reference tothe groups of the periodic table of the elements is to the new IUPACsystem for numbering element groups, as set forth in the Handbook ofChemistry and Physics, 80th Edition (CRC Press, 2000). For use in anoptical waveguide or light concentrator, the nanowires or branchednanowires preferably comprise a fluorescent material, more preferablyone with a high quantum yield (e.g., a quantum yield greater than about5%, 10%, 20%, 50%, or 75%). For nanowires or branched nanowiresabsorbing light with wavelengths between 300 and 3000 nm, the nanowiresor branched nanowires can comprise one or more materials having aband-gap energy between about 0.4 eV and about 4.1 eV.

Each nanowire or branched nanowire can comprise a single material or canbe a heterostructure comprising at least two different and/ordistinguishable materials. The two or more materials can be entirelydifferent (e.g., can have different chemical compositions), or they cancomprise the same base material but have different dopants or differentconcentrations of the same dopant. The different materials can bedistributed at different locations along the long axis of the nanowireor along the long axis of an arm of the branched nanowire, or differentarms of the branched nanowire can comprise different materials.Alternatively, the heterostructures can be core-shell heterostructures,in which a nanowire or branched nanowire comprises a core of a firstmaterial and at least one shell of a second (or third etc.) material,where the materials are distributed radially about the long axis of thenanowire or the long axis of an arm of the branched nanowire. Suitablematerials for a fluorescence efficiency-enhancing shell include e.g.materials having a higher band gap energy than the material forming thenanostructure's core. In addition to having a band gap energy greaterthan that of the core material, suitable materials for the shell canhave e.g. good conduction and valence band offset with respect to thecore material. That is, the conduction band is preferably higher and thevalence band is preferably lower than those of the core material. Forcores that emit energy in the visible range (e.g., CdS, CdSe, CdTe,ZnSe, ZnTe, GaP, GaAs) or near-infrared (e.g., InP, InAs, InSb, PbS,PbSe), a material that has a band gap energy e.g. in the ultravioletrange can be used (e.g., ZnS, GaN, and magnesium chalcogenides, e.g.,MgS, MgSe, and MgTe). For a core that emits in the near-infrared,materials having a band gap energy e.g. in the visible range (e.g. CdSor CdSe) can also be used.

In all embodiments, at least one collector for collecting the waveguidedlight (e.g., a detector, fiber optic cable, photocell, or solar cell) isoptionally operably connected to at least one edge of the core.Optionally, any edges or portions of an edge of the core not occupied bythe collector can be mirrored or silvered.

Any of the embodiments described above can be used to form multilayertandem light concentrators, in which different wavelengths (or differentsingle bands of wavelengths) are collected in different layers andwaveguided, e.g., to photocells or solar cells optimized for thesespecific wavelengths. (As one example, one layer can concentrate nearultraviolet-low wavelength visible light, another layer can concentratevisible-near infrared light, and a third layer can concentrate nearinfrared-infrared light.) Such a multilayer light concentrator comprisesa stack of two or more waveguides as described above. It will be notedthat there need be no physical distinction or delineation between thecladding of successive waveguides in such a stack, as long as thecladding has an index of refraction that is less than the index ofrefraction of each of the cores. The order of the waveguides within thestack is preferably such that the waveguide comprising the nanowires orbranched nanowires absorbing the shortest wavelength light (having thehighest band-gap) is located closest to the light source (e.g., the sun)and the waveguide comprising the nanowires or branched nanowiresabsorbing the longest wavelength light (having the smallest band-gap) islocated farthest from the light source.

Nanostructures Outside the Core

One general class of embodiments is based on the observation, describedin e.g. Lukosz (1981) “Light emission by multipole sources in thinlayers. I. Radiation patterns of electric and magnetic dipoles” J. Opt.Soc. Am. 71,744-754; Lukosz et al. (1977) “Light emission by magneticand electric dipoles close to a plane dielectric interface. II.Radiation patterns of perpendicular oriented dipoles” J. Opt. Soc. Am.67, 1615-1619; Lukosz (1979) “Light emission by magnetic and electricdipoles close to a plane dielectric interface. III. Radiation patternsof dipoles with arbitrary orientation” J. Opt. Soc. Am. 69, 1495-1503;Fattinger et al. (1984) “Optical-environment-dependent lifetimes andradiation patterns of luminescent centers in very thin films” Journal ofLuminescence 31 & 32, 933-935; Lukosz and Kunz (1977) “Light emission bymagnetic and electric dipoles close to a plane interface. I. “Totalradiated power” J. Opt. Soc. Am. 67, 1607-1614; Lukosz and Kunz (1977)“Fluorescence lifetime of magnetic and electric dipoles near adielectric interface” Optics Communications 20:195-199; Chance et al.(1975) “Luminescent lifetimes near multiple interfaces: A quantitativecomparison of theory and experiment” Chem. Phys. Lett. 33:590-592;Lukosz and Kunz (1980) “Changes in fluorescence lifetimes induced byvariable optical environments” Phys. Rev. B 21:4814-4828; Chance et al.(1974) “Lifetime of an emitting molecule near a partially reflectingsurface” J. Chem. Phys. 60:2744-2748; Drexhage (1970) “Influence of adielectric interface on fluorescence decay time” J. Lumin. 1,2:693-701;and Chance et al. (1974) “Lifetime of an excited molecule near a metalmirror: Energy transfer in the Eu³⁺/silver system” J. Chem. Phys.60:2184-2185, that a radiating dipole located on or near a dielectricinterface emits most of its light into the higher index material atangles greater than the critical angle for that interface. Additionally,if the dipole is oriented at an angle substantially normal to thedielectric interface, an even greater percentage of the light is emittedbeyond the critical angle.

This class of embodiments provides a waveguide that comprises acladding, a first core, and a first layer comprising one or morenanostructures. The one or more nanostructures can be nanowires,branched nanowires (e.g., nanotetrapods, tripods, or bipods), ornanocrystals. The first and second surfaces of the first core aresubstantially parallel to each other, permitting the core to act as anefficient waveguide. The first layer is distributed on the first surfaceof the first core, but the core and the layer are not necessarily incontact with each other. The cladding has a first portion distributed onthe second surface of the first core and a second portion distributed onthe first surface of the first layer, but the cladding is notnecessarily in direct contact with these surfaces.

In one embodiment, shown schematically in FIG. 3, the second portion ofcladding 27 is in contact with at least a majority of first surface 21of first layer 20, second surface 22 of the first layer is in contactwith at least a majority of first surface 24 of first core 23, and atleast a majority of second surface 25 of the first core is in contactwith first portion of cladding 26. In this embodiment, the claddingtypically has a refractive index less than the refractive index of thecore. In a preferred embodiment, the one or more nanostructures absorblight impinging on the top or bottom surface of the waveguide and emitlight, and a majority of the emitted light is emitted into the firstcore at an angle greater than the critical angle for the specificcore-first layer combination. A majority of the emitted light is thuswaveguided and directed toward at least one edge of the core. This typeof waveguide can be an efficient light concentrator, since the waveguidecan absorb light essentially isotropically over a large surface area anddirect it to an edge which has a much smaller area.

Alternatively, the first layer and the first core can be separated,e.g., by a layer of a material having a refractive index between that ofthe first layer and first core.

In other embodiments, the waveguide further comprises a second core. Thesecond core has two substantially parallel surfaces and is locatedbetween, but is not necessarily in contact with, the first layer and thesecond portion of the cladding. Typically, the first and second corescomprise the same material, although they can be formed of differentmaterials. In one specific embodiment, shown schematically in FIG. 4,the second portion of the cladding 37 is in contact with the top surface39 of the second core 38, the bottom surface 40 of the second core is incontact with the first surface 31 of the first layer 30, the secondsurface 32 of the first layer is in contact with the first surface 34 ofthe first core 33, and the second surface 35 of the first core is incontact with the first portion of the cladding 36. In this embodiment,the first layer and the cladding portions each have a refractive indexless than that of the first and second cores.

In another embodiment, a waveguide comprising a second core furthercomprises a second layer located between the second core and the firstlayer and a third layer located between the first layer and the firstcore. The second and third layers each have an index of refraction thatis greater than the refractive index of the first layer but less thanthe refractive index of the first and second cores. As in the exampleillustrated in FIG. 5, the second layer 51 is typically in contact withthe second core 53 and the first layer 50, and the third layer 52 istypically in contact with the first layer 50 and the first core 54. Inthis embodiment, the cladding portions 55 (shown in contact with thesecond core 53) and 56 (shown in contact with the first core 54)preferably have an index of refraction less than that of the first andsecond cores. The second and third layers can be the same or differentmaterials, and preferably are formed from a material that issubstantially nonabsorbing and scattering at the relevant wavelengths(e.g., greater than about 300 nm, or between about 300 or 400 nm andabout 10 or 20 micrometers). In this embodiment, the first layer istypically a thin layer having a high concentration of nanostructures.The high concentration of nanostructures in the first layer permitsefficient absorption of light, which is re-emitted primarily into thesurrounding second and third layers and then passes into the cores.Removal of the light from the nanostructure layer reduces losses fromscattering and reabsorption of the emitted light by the nanostructures.

In any of the above embodiments, the first layer can consist e.g., of aplurality of substantially pure nanostructures, e.g., nanostructureswithout a surrounding matrix. Such a layer can be formed, for example,by spin-casting or otherwise depositing a solution of nanostructures ina solvent on the surface of a core or other layer and then evaporatingthe solvent. Alternatively, the first layer can comprise e.g., one ormore nanostructures in a matrix (the first layer can be ananocomposite). Preferred matrices have a relatively low index ofrefraction, since the first layer preferably has a refractive index thatis less than that of the first (and second, if present) core. The matrixcan be, for example, a small molecule or molecular matrix or a matrixcomprising at least one polymer or glass. A number of small molecules,glasses, and polymers (e.g., organic, inorganic, and organometallicpolymers) are known in the art (e.g., those mentioned and/or referencedherein). In a preferred embodiment, the first layer comprises one ormore nanostructures in a matrix comprising a polysiloxane, preferablypolydimethylsiloxane (PDMS). In another preferred embodiment, the matrixcomprises an inorganic glassy material such as SiO₂ or TiO₂. Thenanostructures can but need not be dispersed uniformly throughout thefirst layer, e.g., the nanostructures can be located primarily in aportion of the first layer that is adjacent to one of the cores.

The cladding comprises a material that has a lower refractive index thanthe core, e.g., a lower refractive index solid, liquid, or gas, e.g.,air, a low index plastic or polymer film or sheet, or any otherconvenient material having a smaller index of refraction than the core.The first and second portions of the cladding need not comprise the samematerial.

To optimize the number of photons that can be waveguided and collected,the first core is preferentially substantially nonabsorbing andnonscattering with respect to relevant wavelengths (including, forexample, those wavelengths emitted by the nanowires or branchednanowires distributed on the core). For a waveguide for use in a solarconcentrator, for example, the first core is preferably substantiallynonabsorbing and nonscattering to light in the visible, near-infrared,and infrared range, and thus is preferably substantially nonabsorbingand nonscattering with respect to wavelengths greater than about 300 nm(e.g., greater than about 300 or 400 nm and less than about 10 or 20micrometers). The larger the index difference at a dielectric interface,the smaller the critical angle for that interface and the larger thenumber of photons emitted into the core by the nanostructures that arewaveguided by total internal reflection and collected. Thus the use ofhigh index cores is advantageous. For example, the first core can havean index of refraction greater than about 1.35, greater than about 2.5,greater than about 3.3, or even greater than 4. Preferably, the firstcore has an index of refraction between about 1.35 and about 4. In someinstances, a physical property of the first core such as e.g. itsmechanical strength can be another criterion used to select anappropriate core material, since in some embodiments the waveguide (andtypically also the core) is a flat sheet, which can be of considerablesize. (As will be evident, weaker materials can still be formed intolarge sheets, e.g. with the provision of suitable physical support.)Similar considerations apply to the second core.

The first (or second) core can comprise e.g. a glass, a polymer, anorganic polymer, an inorganic polymer, an organometallic polymer, asmall molecule or molecular matrix, a gel, a liquid, a crystal, apolycrystal, or a mesoporous matrix; a number of these are known (e.g.,the example polymers and small molecules mentioned and/or referencedherein). Example materials for the first (or second) core include butare not limited to acrylic and poly(methyl methacrylate); a large numberof other suitable materials are known in the art.

To reduce losses in the waveguide, the first layer is preferablysubstantially nonscattering with respect to light at relevantwavelengths (e.g., the wavelengths absorbed and emitted by thenanostructures). For example, for a waveguide for use in a solarconcentrator, the first layer is preferably substantially nonscatteringto light in the visible, near-infrared, and infrared range, and thus ispreferably substantially nonscattering with respect to wavelengthsgreater than about 300 nm (e.g., greater than about 300 or 400 nm andless than about 10 or 20 micrometers). The first layer is alsopreferably substantially nonabsorbing with respect to the wavelength orwavelengths of light emitted by the nanostructures in the layer.

In a preferred embodiment, the first layer has a thickness less thanabout one wavelength of the light emitted by the one or morenanostructures. For a waveguide for use with visible light, for example,the first layer can have a thickness less than about 1000 nm (e.g., fornear-infrared light), less than about 900 nm, less than about 800 nm,less than about 700 nm, less than about 600 nm, less than about 500 nm(e.g., for green light), or less than about 400 nm. The thickness of thefirst layer affects the efficiency of the waveguide because the effecton which this class of embodiments is based, the emission of light by aradiating dipole into the higher index material at a dielectricinterface, decreases as the dipole is moved away from the interface andhas no effect beyond the distance of about one wavelength of the lightemitted. In most cases, a layer substantially thinner than this can beused to absorb substantially all the incident light, so this does notresult in a performance limitation.

The first layer can comprise one or more nanostructures (e.g.,nanowires, branched nanowires, or nanocrystals), or a plurality ofnanostructures. The one or more nanostructures can be fabricated fromessentially any convenient materials. For example, the nanostructurescan comprise a semiconducting material, for example a materialcomprising a first element selected from group 2 or from group 12 of theperiodic table and a second element selected from group 16 (e.g., ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS,CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and like materials); amaterial comprising a first element selected from group 13 and a secondelement selected from group 15 (e.g., GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, and like materials); a material comprising a group 14element (Ge, Si, and like materials); a material such as PbS, PbSe,PbTe, AlS, AlP, and AlSb; or an alloy or a mixture thereof. Preferredmaterials include CdTe, InP, InAs, PbS, PbSe, PbTe, CdSe, and CdS. Foruse in an optical waveguide or light concentrator, the nanostructurespreferably comprise a fluorescent material, more preferably one with ahigh quantum yield (e.g., a quantum yield greater than about 5%, 10%,20%, 50%, or 75%). For nanostructures absorbing light with wavelengthsbetween 300 and 3000 nm, the nanostructures can comprise one or morematerials having a band-gap energy between about 0.4 eV and about 4.1eV.

Each nanostructure can comprise a single material or can be aheterostructure comprising at least two different and/or distinguishablematerials. The two or more materials can be entirely different (e.g.,can have different chemical compositions), or they can comprise the samebase material but have different dopants or different concentrations ofthe same dopant. The heterostructures can be core-shellheterostructures, in which a nanowire, branched nanowire, or nanocrystalcomprises a core of a first material and at least one shell of a second(or third etc.) material, where the materials are distributed radiallyabout the long axis of the nanowire, the long axis of an arm of thebranched nanowire, or the center of the nanocrystal. (Suitable materialsfor a fluorescence efficiency-enhancing shell include e.g. materialshaving a higher band gap energy than the material forming thenanostructure's core. In addition to having a band gap energy greaterthan that of the core material, suitable materials for the shell canhave e.g. good conduction and valence band offset with respect to thecore material. That is, the conduction band is preferably higher and thevalence band is preferably lower than those of the core material. Forcores that emit energy in the visible range (e.g., CdS, CdSe, CdTe,ZnSe, ZnTe, GaP, GaAs) or near-infrared (e.g., InP, InAs, InSb, PbS,PbSe), a material that has a band gap energy e.g. in the ultravioletrange can be used (e.g., ZnS, GaN, and magnesium chalcogenides, e.g.,MgS, MgSe, and MgTe). For a core that emits in the near-infrared,materials having a band gap energy e.g. in the visible range (e.g. CdSor CdSe) can also be used.) Alternatively, the different materials canbe distributed at different locations within the nanostructure. Forexample, the different materials can be at different locations along thelong axis of a nanowire or along the long axis of an arm of a branchednanowire, or different arms of a branched nanowire can comprisedifferent materials.

The size of the nanostructures can be varied; for example, to controlthe wavelengths emitted by the nanostructures. In certain embodiments,the waveguides comprise nanowires having an average diameter betweenabout 2 nm and about 100 nm (e.g., between about 2 nm and about 50 nm,or between about 2 nm and about 20 nm). For certain embodiments (e.g.,embodiments in which the nanowires are oriented perpendicular to thesurface of a thin first layer), nanowires with shorter aspect ratios arepreferred; thus, in some embodiments the nanowires preferably have anaspect ratio between about 1.5 and about 100, e.g. between about 5 andabout 30. In certain embodiments, the waveguides comprise nanocrystalshaving an average diameter between about 1.5 nm and about 15 nm (e.g.,between about 2 nm and about 5 nm, between about 5 nm and about 10 nm,or between about 10 nm and about 15 nm). The nanocrystals typically havean aspect ratio between about 0.1 and about 1.5, e.g., between about 0.5and about 1.5, preferably between about 1 and about 1.5.

In some embodiments, the first layer comprises a plurality of nanowires,which can be either randomly or substantially nonrandomly oriented. In apreferred embodiment, the orientation of the nanowires is substantiallynonrandom, with the vector average of the nanowires' orientations havinga nonzero component perpendicular to a surface of the first core.Preferably at least 1%, more preferably at least 10%, and mostpreferably at least 50% of all the nanowires in the first layer aresubstantially nonrandomly oriented in this manner, since the greater thedegree of orientation perpendicular to the surface of the core, thegreater the efficiency of capture of the re-emitted light. In oneembodiment, a majority of the nanowires each has its long axis orientedmore nearly perpendicular to than parallel to a surface of the firstcore. In one specific embodiment, the nanowires within the first layerform a liquid crystal phase, in which each nanowire has its long axisoriented substantially perpendicular to a surface of the first core. Theuse of a liquid crystal phase results in a first layer that absorbslight from substantially all directions equally but emits lightprimarily into the adjacent higher index material at angles greater thanthe critical angle, providing a highly efficient light concentrator (asmuch as eight times more efficient than a similar concentrator where thenanowires are embedded inside the waveguide core).

In other embodiments, the first layer comprises a thin film comprisingor consisting of nanocrystals. The nanocrystals emit more light into theadjacent higher index material (e.g., the core) at an angle greater thanthe critical angle, but the effect is not as pronounced as for a firstlayer comprising nanowires (particularly substantially nonrandomlyoriented nanowires). A waveguide containing nanocrystals distributed ona core is, however, still more efficient than a waveguide containingquantum dots embedded in the core. If the nanocrystals have a uniquecrystal axis, efficiency can be increased by substantially nonrandomlyorienting the nanocrystals such that a vector average of thenanocrystals orientations has a nonzero component perpendicular to asurface of the core. The greater the degree of orientation perpendicularto the surface of the core, the greater the efficiency of capture of thereemitted light.

In all embodiments, at least one collector for collecting the waveguidedlight (e.g., a detector, fiber optic cable, photocell, or solar cell) isoptionally operably connected to at least one edge of the first core. Inembodiments in which the waveguide comprises a second core, a secondcollector can optionally be connected to at least one edge of the secondcore. Optionally, any edges or portions of an edge of the core or coresnot occupied by the collector can be mirrored or silvered.

Any of the embodiments described above can be used to form multilayertandem light concentrators, in which different wavelengths (or differentsingle bands of wavelengths) are collected in different layers andwaveguided, e.g., to photocells or solar cells optimized for thesespecific wavelengths. (As one example, one layer can concentrate nearultraviolet-low wavelength visible light, another layer can concentratevisible-near infrared light, and a third layer can concentrate nearinfrared-infrared light.) Such a multilayer light concentrator comprisesa stack of two or more waveguides as described above. It will be notedthat there need be no physical distinction or delineation between thecladding of successive waveguides in such a stack, as long as thecladding has an index of refraction that is less than the index ofrefraction of each of the cores. The order of the waveguides within thestack is preferably such that the waveguide comprising thenanostructures absorbing the shortest wavelength light (having thehighest band-gap) is located closest to the light source (e.g., the sun)and the waveguide comprising the nanostructures absorbing the longestwavelength light (having the smallest band-gap) is located farthest fromthe light source. In an alternative embodiment, a multi-layerconcentrator can be fabricated such that it is symmetrically structuredwith the lowest energy bandgap waveguide in the center of the stack andincreasing bandgap energies as the waveguide layers extend above andbelow the center layer(s). In this embodiment, light can impinge on thestack from either side with equal efficiency. A multilayer lightconcentrator can also be assembled as a stack of waveguides in which atleast one waveguide comprises nanowires within the core and at least onewaveguide comprises nanostructures distributed on the core (eachwaveguide can include any of the variations described above).

Emission Properties of Nanostructures

Note that one of the characteristics of all of the previous embodimentsis that the nanostructures typically absorb light over a broad spectralrange, e.g. compared to a dye molecule with emission at comparablewavelengths. The wavelength range over which light is absorbed can alsobe e.g. substantially broader than the range over which light isemitted. For example, a nanostructure can emit light over a narrowspectral range, e.g., less than about 60 nm, preferably less than about40 nm or about 30 nm.

An additional characteristic of previous embodiments is that, fornanostructures, absorption typically increases in strength the fartherin energy the light is from the emission wavelength (at shorterwavelengths only). For example, a nanorod with emission at 700 nm canabsorb 300 nm light more strongly than 400 nm light, which can be morestrongly absorbed than 500 nm light, which can be more strongly absorbedthan 600 nm light. This is very different than what is observed for dyemolecules, e.g. in a solar concentrator. For dyes, the absorption peaksat an energy very close to the emission energy and then decreasesquickly. As a result, conversion of light impinging on a dye moleculesolar concentrator is very inefficient, since most of the wavelengthsare not strongly absorbed.

FIG. 6 depicts examples of absorption and emission spectra for a dye anda nanostructure. Panel A depicts absorption (61) and emission (62) bythe organic dye fluorescein. Panel B depicts absorption (63) andemission (64) by spherical CdSe nanocrystals with an aspect ratio ofabout 1.1 and an average diameter of about 2.3 nm. Other dyes andnanostructures have spectra with comparable features, e.g., generallycomparable shape.

Orientation of Nanowires and Other Nanostructures

Substantially nonrandom orientation of nanostructures (particularlynanowires) is frequently desirable, e.g., within a composite material.For example, as described above, waveguides and light concentratorscomprising nonrandomly oriented nanostructures can be more efficientthan similar waveguides or concentrators comprising randomly orientednanostructures.

Composites in which Matrix Orients Nanowires

One aspect of the present invention provides a composite materialcomprising a plurality of nanowires and a small molecule or molecularmatrix or a matrix comprising at least one polymer, where the matrix (orthe components and/or precursors of the matrix) is used to orient thenanowires (to produce a substantially nonrandom distribution of nanowireorientations). The nanowires can be fabricated of essentially anyconvenient material (e.g., a semiconducting material, a ferroelectricmaterial, a metal, etc.) and can comprise essentially a single materialor can be heterostructures.

In one embodiment, the matrix comprises a polymer that exhibits a liquidcrystal phase. A combination of polymer and nanowires is selected, suchthat the mixture also exhibits a liquid crystal phase. For example, asolution of nanowires (e.g., at a high concentration of nanowires) andpolymer can be mixed together and placed in a small electric field toorient them in the desired direction. Typically, the polymer and thenanowires have comparable lengths; therefore, shorter nanowires arepreferred (e.g., nanowires having lengths less than about 100 nm, lessthan about 50 nm, or less than about 20 nm). Liquid crystal polymersinclude, e.g., poly(m-phenylene isophthalamide), poly(p-benzamide),poly(alkyl isonitriles), polyisocyanates, and a number of otherpolymers, see e.g., Dietrich Demus, John W. Goodby, George W. Gray, HansW. Spiess, and Volkmar Vill (1998) Handbook of Liquid Crystals, Handbookof Liquid Crystals: Four Volume Set, John Wiley and Sons, Inc.

Methods for Orienting Nanowires

As described in the preceding section, a liquid crystalline polymer orother polymeric or small molecule or molecular matrix can be used orientnanowires. Other methods for orienting nanowires are known to those ofskill in the art. Liquid crystal phases of nanowires (e.g., nanorods)are described in, for example, Li et al (2002) “Semiconductor nanorodliquid crystals” Nano Letters 2: 557-560. Nanorods aligned along thestretching direction in stretched polymer films are described in e.g.Peng et al (2000) “Shape control of CdSe nanocrystals” Nature 404:59-61. As another example, aligned nanowires can be grown substantiallyin situ as a field of oriented structures that are subsequentlyintegrated into the polymer matrix. Methods of fabricating such fieldsof aligned nanostructures are described, e.g., in Published U.S. PatentApplication Nos. 2002/0172820 and 2002/0130311. Methods of orientingnanowires also include electric field assisted orientation of nanowires(e.g., nanorods), e.g., applying an electric field to cause magneticnanorods to orient in a desired direction, in a polymer matrix whichoptionally can be hardened to maintain the orientation.

Orientation of Other Nanostructures

Other nanostructures can also be oriented to some degree. See e.g.,Alivisatos (2000) “Naturally Aligned Nanocrystals” Science, 289, 736.For example, nanotetrapods can be self-orienting. When deposited on asurface, the nanotetrapods typically contact the surface via three arms;the fourth arm is perpendicular to the surface. In some cases,nanocrystals will spontaneously align with their unique crystal axisoriented vertically when they are packed at high density on a surface.In certain preferred embodiments, such nanocrystals are substantiallyfaceted in shape such that the lowest energy packing state is one inwhich the non-symmetric unique axis of every nanocrystal is orientedvertically.

Methods for Verifying Orientation

The extent to which the nanostructures in a collection of nanostructuresare ordered or nonrandomly oriented can be determined through variousmethods. For example, electron microscopy (e.g., SEM, TEM), atomic forcemicroscopy (AFM), or scanning tunneling microscopy (STM) can readily beperformed, e.g., on cross-sections of a nanocomposite material. Inaddition, in many cases, optical microscopy such as polarization or DICmicroscopy can be used to determine the average orientation of nanowiresor nanocrystals within a material. Electrostatic force microscopy canalso be used in some cases to determine orientation due to an intrinsicdipole in many nanocrystals and nanowires oriented along their uniquecrystal axis.

Polysiloxane Nanocomposites

One aspect of the invention provides a composite material comprising oneor more nanostructures and a polymeric matrix comprising a polysiloxane(e.g., a composite material comprising one or more nanostructures and apolymeric matrix consisting of a polysiloxane). A polysiloxane is aninorganic polymer whose backbone comprises alternating silicon andoxygen atoms; each silicon has two side groups. These groups can bee.g., a hydrogen atom, any organic group, or any alkyl group. Examplepolysiloxanes include but are not limited to polydimethylsiloxane(PDMS), polymethylphenylsiloxane, polydiphenylsiloxane, andpolydiethylsiloxane. A polysiloxane can also be a copolymer comprisingat least two different types of siloxane monomers, e.g.,dimethylsiloxane and methylhydrogensiloxane (in which one of the sidegroups is a methyl group and the other side group is a hydrogen). Inpreferred embodiments, the composite comprises one or morenanostructures in a matrix comprising polydimethylsiloxane (PDMS) or ina matrix comprising a copolymer between dimethylsiloxane and anothersiloxane (e.g., methylhydrogensiloxane). The polysiloxane matrix cancomprise e.g. linear or cross-linked polysiloxane oligomers. The matrixcan be e.g. substantially free of silicates. The polymeric matrix isdistinct from ORMOSIL (organically modified silicate) matrices.

PDMS and other siloxane polymers are commercially readily available. Forexample, kits comprising vinyl-terminated dimethylsiloxane oligomers(FIG. 7, Panel A), siloxane cross-linkers (polysiloxane oligomers wherethe monomers are usually dimethylsiloxane but occasionallymethylhydrogensiloxane, Panel B), and a catalyst (which catalyzes theaddition of an SiH bond across a vinyl group to form an Si—CH₂—CH₂—Silinkage, as illustrated in Panel C) are generally commerciallyavailable, e.g., from Dow Corning (SYLGARD 184™, www.dowcorning.com).

The nanostructures can be e.g., nanowires, nanocrystals, branchednanowires (e.g., nanotetrapods, tripods, or bipods), nanoparticles,teardrops, or any other desired nanostructures. The nanostructures canbe fabricated from essentially any convenient material. Thenanostructures can comprise e.g. one or more metals (e.g., Au, Ag, Ni,Cu, Zn, or Pt), or an alloy or mixture thereof The nanostructures cancomprise e.g., a ferroelectric or ferroelectric ceramic material,including, for example, a perovskite-type material (including but notlimited to BaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃, LiTiO₃, LiTaO₃, orLiNbO₃, or a material derived from these, for example,Ba_((1-x))Ca_(x)TiO₃ where x is between 0 and 1, or PbTi_((1-x))Zr_(x)O₃where x is between 0 and 1); a KDP-type material (e.g., KH₂PO₄, KD₂PO₄,RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄, or GeTe, or a material derived from these);a TGS-type material (e.g., tri-glycine sulfate, tri-glycine selenate, ora material derived from these); or a mixture thereof As another example,the nanostructures can comprise a semiconducting material, for example,a material comprising a first element selected from group 2 or fromgroup 12 of the periodic table and a second element selected from group16 (e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe,MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and likematerials); a material comprising a first element selected from group 13and a second element selected from group 15 (e.g., GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, and like materials); a material comprising a group14 element (Ge, Si, and like materials); a material such as PbS, PbSe,PbTe, AlS, AlP, and AlSb; or an alloy or a mixture thereof. For use inan optical waveguide or light concentrator, for example, thenanostructures preferably comprise a fluorescent material, morepreferably one with a high quantum yield (e.g., a quantum yield greaterthan about 5%, 10%, 20%, 50%, or 75%). For nanostructures absorbinglight with wavelengths between 300 and 3000 nm, the nanostructures cancomprise one or more materials having a band-gap energy between about0.4 eV and about 4.1 eV. Each nanostructure can comprise a singlematerial or can be a heterostructure comprising at least two differentand/or distinguishable materials.

The composite material can further comprise at least one surfactant orat least one solvent. For example, the composite can comprise asurfactant used to assist in uniformly dispersing the nanostructuresthroughout the polysiloxane matrix. The surfactant can interact with thesurface of the nanostructures and with the polysiloxane monomers orpolymer. In addition, the surfactant can be attached to the surface ofthe nanostructures by ionic or covalent bonds, or other molecular forcesand/or to the polymer by ionic or covalent bonds, or other molecularforces.

The polysiloxane nanocomposite of this invention can be formed into anumber of shaped articles. A light-emitting diode (LED), laser,waveguide (e.g., as described herein), or amplifier, for example, cancomprise a polysiloxane-containing composite. See e.g. US 20010046244entitled “Optical amplifiers and lasers” by Klimov et al. In particular,it is possible to form a variety of optically relevant structures suchas waveguides and optical gratings using polysiloxane templated onto asecondary structure (as is known in the art). Such structures will begreatly improved by the inclusion of one or more nanostructures toprovide additional optical, electronic, or other functionality to anotherwise passive material. In addition, it is known that polysiloxanessuch as PDMS are useful in the formation of microfluidic devices. Thecomposite of the present invention allows the incorporation of activeoptical components into such devices.

Additional Nanocomposites

One class of embodiments provides a composite material that compriseseither a small molecule or molecular matrix or a matrix comprising atleast one organic polymer or an inorganic glass and one or more branchednanowires or one or more inorganic nanowires (or a combination thereof).The inorganic nanowires have an aspect ratio greater than about 10(e.g., greater than about 15, greater than about 20, or greater thanabout 50) and can be either semiconducting or ferroelectric. Thebranched nanowires can be nanotetrapods, for example, or can be otherbranched structures such as e.g. tripods or bipods.

In some embodiments, the composite comprises a plurality of inorganicnanowires, which can be either randomly or substantially nonrandomlyoriented. In some preferred embodiments, the composite material isformed into a thin film within which the orientation of the inorganicnanowires is substantially nonrandom. For example, at least 1% (e.g., atleast 10%, or at least 50%) of all the nanowires in the film can besubstantially nonrandomly oriented. The thin film can be eithersubstantially free of strain, or it can be highly strained (for example,if the nonrandom orientation of the nanowires is achieved by stretchingthe film). In one embodiment, the composite is formed into a thin filmwithin which a majority of the nanowires are oriented such that eachnanowire has its long axis substantially parallel to a surface of thefilm. In another embodiment, the composite is formed into a thin filmwithin which a majority of the nanowires are oriented such that eachnanowire has its long axis more nearly perpendicular than parallel to asurface of the film. In yet another embodiment, the composite is formedinto a thin film within which a majority of the nanowires are orientedsuch that each nanowire has its long axis substantially perpendicular toa surface of the film.

The inorganic nanowires can be fabricated from a ferroelectric orferroelectric ceramic material. For example, the inorganic nanowires cancomprise a perovskite-type material (including but not limited toBaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃, LiTiO₃, LiTaO₃, or LiNbO₃, or amaterial derived from these, for example, Ba_((1-x))Ca_(x)TiO₃ where xis between 0 and 1, or PbTi_((1-x))Zr_(x)O₃ where x is between 0 and 1);a KDP-type material (e.g., KH₂PO₄, KD₂PO₄, RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄,or GeTe, or a material derived from these); a TGS-type material (e.g.,tri-glycine sulfate, tri-glycine selenate, or a material derived fromthese); or a mixture thereof Alternatively, the inorganic nanowires canbe fabricated from a semiconducting material. For example, the nanowirescan comprise a material comprising a first element selected from group 2or from group 12 of the periodic table and a second element selectedfrom group 16 (e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe,MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, andlike materials); a material comprising a first element selected fromgroup 13 and a second element selected from group 15 (e.g., GaN, GaP,GaAs, GaSb, InN, InP, InAs, InSb, and like materials); a materialcomprising a group 14 element (Ge, Si, and like materials); a materialsuch as PbS, PbSe, PbTe, AlS, AlP, and AlSb; or an alloy or a mixturethereof.

The branched nanowires can be fabricated from essentially any convenientmaterial. For example, the branched nanowires can comprise a metal(e.g., Ag, Au, Ni, Cu, Zn, or Pt) or a ferroelectric or semiconductingmaterial (e.g., those listed in previous sections).

In certain embodiments, the composite comprises a small molecule ormolecular matrix. A variety of such matrices are known in the art,comprising e.g., molecular organics such as those used in OLEDs;N,N′-diphenyl-N,N′-bis (3-methylphenyl)-(1,1′biphenyl)-4,4′-diamine)(TPD); (3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole)(TAZ); tris-(8-hydroxyquinoline) aluminum (Alq₃); benzoic acid; phthalicacid; benzoin; hydroxyphenol; nitrophenol; chlorophenol; chloroaniline;or chlorobenzoamide. Preferred small molecules have at least one polarfunctional group and have a high enough melting point that they aresolid at room temperature. In other embodiments, the matrix comprises aninorganic glass. A variety of such glasses are known in the art, e.g.,SiO₂ or TiO₂.

In other embodiments, the composite comprises a matrix comprising atleast one organic polymer. A wide variety of such polymers is known tothose of skill in the art (see e.g., Dietrich Demus, John W. Goodby,George W. Gray, Hans W. Spiess, and Volkmar Vill (1998) Handbook ofLiquid Crystals, Handbook of Liquid Crystals: Four Volume Set, JohnWiley and Sons, Inc.; Johannes Brandrup (1999) Polymer Handbook, JohnWiley and Sons, Inc.; and Charles A. Harper (2002) Handbook of Plastics,Elastomers, and Composites, 4^(th) edition, McGraw-Hill). Examplesinclude thermoplastic polymers (e.g., polyolefins, polyesters,polysilicones, polyacrylonitrile resins, polystyrene resins, polyvinylchloride, polyvinylidene chloride, polyvinyl acetate, orfluoroplastics); thermosetting polymers (e.g., phenolic resins, urearesins, melamine resins, epoxy resins, polyurethane resins); engineeringplastics (e.g., polyamides, polyacrylate resins, polyketones,polyimides, polysulfones, polycarbonates, polyacetals); and liquidcrystal polymers, including main chain liquid crystal polymers (e.g.,poly(hydroxynapthoic acid)) and side chain liquid crystal polymers(e.g., poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>). Certainembodiments include conductive organic polymers; see e.g. T. A.Skatherin (ed.) Handbook of Conducting Polymers I. Examples include butare not limited to poly(3-hexylthiophene) (P3HT), poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV), poly(phenylenevinylene) (PPV), and polyaniline.

The diameter of the inorganic nanowires can be varied, for example, tocontrol the wavelength emitted by fluorescent nanowires. The diameter ofthe nanowires is preferably between about 2 nm and about 100 nm, morepreferably between about 2 nm and about 5 nm or between about 10 nm andabout 50 nm. The length of the nanowires can also be varied. In certainembodiments, the inorganic nanowires have an aspect ratio between about10 and about 10,000 (e.g., between about 20 and about 10,000, betweenabout 50 and about 10,000, or between about 100 and about 10,000).

The composite materials comprising ferroelectric nanowires orferroelectric branched nanowires have a number of uses. For example, thenanocomposites can be used in data storage media, where information isencoded by flipping the ferroelectric state of individual nanowires ornanoparticles to write bits. As another example, in embodiments wherethe composite has a high dielectric constant, the composite can be usedas an insulator (e.g., formed into thin insulating films such as thoseprinted on circuit boards). The composite materials comprisingsemiconducting nanowires or semiconducting branched nanowires also havea number of uses. For example, a composite comprising semiconductingnanowires or branched nanowires and e.g. a conductive polymer or smallmolecule or molecular matrix can be used in a photovoltaic device, wherethe nanowires conduct the electrons and the polymer or small molecule ormolecular matrix is the hole-conducting element.

Nanocomposites with a Blocking Layer

In many applications (e.g., in photovoltaic devices, e.g., devices suchas certain of those described in U.S. patent application Ser. No.60/421,353, filed Oct. 25, 2002, U.S. Provisional Patent Application No.60/452,038, filed Mar. 4, 2003, and U.S. patent application Ser. No.10/656,802, filed Sep. 4, 2003), at least one blocking layer isoptionally used to restrict the movement of charges (e.g., to preventthe movement of electrons or holes in a particular direction, through aparticular region of a device, or to a particular electrode). A blockinglayer is a layer of some material either organic or inorganic, which haseither: a high hole conductivity and high electron resistance to allowhole flow and prevent electron flow, or a high electron conductivity andhigh hole resistance to allow electron flow and prevent hole flow.Blocking layers can be incorporated e.g. onto either one or both sidesof the active layer (e.g., a nanocomposite layer) of a photovoltaicdevice. For example, one or more blocking layers can be used to preventa nanostructure photovoltaic cell from beginning to short out uponstrong illumination and to assist in the creation of a high carrierdensity within nanostructures extending across the active layer. Asanother example, one or more blocking layers can be used to concentratecharges in an active region of a nanostructure-base LED to improvecharge recombination within the nanocrystals.

Accordingly, one aspect of the invention provides a composite materialcomprising a plurality of nanostructures and a small molecule ormolecular matrix, a glassy or crystalline inorganic matrix, or a matrixcomprising at least one polymer, where the composite is distributed on afirst layer of a material that conducts substantially only electrons orsubstantially only holes.

The composite and the first layer can be in contact with each other(e.g., over all or most of a surface of the first layer), or can beseparated, for example, by a second layer comprising a conductivematerial. The second layer can conduct electrons, holes, or both.

The first layer can be distributed on an electrode. The first layer canbe in contact with the electrode (e.g., over all or most of a surface ofthe first layer), or can be separated, for example, by a layer of anonconductive material or by a third layer comprising a conductivematerial. The third layer can conduct electrons, holes, or both.

The nanostructures can be of any desired type (e.g., nanowires, branchednanowires, nanocrystals, or nanoparticles) and can be fabricated ofessentially any convenient material (e.g., any metal or semiconductingor ferroelectric material such as those described herein), and cancomprise a single material or can be heterostructures. The smallmolecule or molecular matrix can comprise e.g., molecular organics suchas those used in OLEDs; N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′biphenyl)-4,4′-diamine) (TPD);(3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole) (TAZ);tris-(8-hydroxyquinoline) aluminum (Alq₃); benzoic acid; phthalic acid;benzoin; hydroxyphenol; nitrophenol; chlorophenol; chloroaniline;chlorobenzoamide; or others known in the art. The glassy matrix cancomprise any glasses known in the art, e.g., an inorganic glass, e.g.,SiO₂ or TiO₂. The crystalline inorganic matrix can be any of those knownin the art, e.g., Al₂O₃. The polymeric matrix can comprise e.g., aninorganic polymer (e.g., a polysiloxane, a polycarbonessiloxane (acopolymer of siloxane and carborane), or a polyphosphazene), anorganometallic polymer (e.g., a ferrocene polymer, a platinum polymer,or a palladium polymer), or an organic polymer (for example, athermoplastic polymer (e.g., a polyolefin, a polyester, a polysilicone,a polyacrylonitrile resin, a polystyrene resin, polyvinyl chloride,polyvinylidene chloride, polyvinyl acetate, or a fluoroplastic), athermosetting polymer (e.g., a phenolic resin, a urea resin, a melamineresin, an epoxy resin, a polyurethane resin, an engineering plastic, apolyamide, a polyacrylate resin, a polyketone, a polyimide, apolysulfone, a polycarbonate, or a polyacetal), a liquid crystalpolymer, including a main chain liquid crystal polymer (e.g.,poly(hydroxynapthoic acid) or a side chain liquid crystal polymer (e.g.,poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>, or a conductingpolymer (e.g., poly(3-hexylthiophene) (P3HT), poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV), poly(phenylenevinylene) (PPV), or polyaniline). See e.g., Dietrich Demus, John W.Goodby, George W. Gray, Hans W. Spiess, and Volkmar Vill (1998) Handbookof Liquid Crystals, Handbook of Liquid Crystals: Four Volume Set, JohnWiley and Sons, Inc.; Johannes Brandrup (1999) Polymer Handbook, JohnWiley and Sons, Inc.; Charles A. Harper (2002) Handbook of Plastics,Elastomers, and Composites, 4^(th) edition, McGraw-Hill; T. A. Skatherin(ed.) Handbook of Conducting Polymers I; and Ronald Archer Inorganic andOrganometallic Polymers for other examples.

The first layer can comprise any convenient material that conductssubstantially only electrons or substantially only holes. For example,3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) andtris-(8-hydroxyquinoline) aluminum (Alq₃) conduct electrons but blocksholes, whileN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD) transports holes. The first layer can be created, for example, byusing a combination of evaporative, spray coating, or spin-coatingprocesses. The evaporation of small molecules and condensation onto asubstrate allows for finely controlled deposition and the design ofcomplex layered structures that can be engineered to balance carrierconduction and optimize charge injection into one of the layers.

The second and/or third layer can comprise any convenient conductivematerial. For example, the second and/or third layers can comprise ametal, a semiconductor, or other material that conducts both electronsand holes, or can comprise a hole or electron conducting material (e.g.,TAZ, Alq₃, or TPD).

The first, second, third, and nanocomposite layers can be formed e.g.,by spin coating, evaporating, or printing (ink-jet, screen, or roll toroll) one layer on top of the other, and then repeating the process,curing or otherwise protecting the lower layers at each step, or by anycombination of such methods. In addition, inorganic blocking layers suchas TiO2 can be formed by vapor deposition of gaseous precursors ornanoparticles of the inorganic material followed by annealing.

Nanocomposites Supporting Charge Separation or Recombination

When light absorbed by the nanostructures in a nanocomposite results inthe formation of an electron-hole pair, the electron and hole can eitherrecombine or remain separated. Recombination of an electron and a holeresults in luminescence (light emission). This phenomenon is useful inthe creation of displays, LEDs, etc. comprising nanocomposites,particularly since the wavelength of light emitted can be controlled,for example, by choosing a nanostructure material having an appropriateband-gap. Recombination of electrons and holes is undesirable in otherapplications, however. In nanocomposites used in photovoltaic devices,for example, the electron and hole preferably do not recombine butrather travel to opposite electrodes. See also, e.g., U.S. patentapplication Ser. No. 60/421,353, filed Oct. 25, 2002, U.S. ProvisionalPatent Application No. 60/452,038, filed Mar. 4, 2003, and U.S. patentapplication Ser. No. 10/656,802, filed Sep. 4, 2003. The presentinvention provides nanocomposite materials that support chargerecombination and other materials that support charge separation.

In one class of embodiments, a composite material that can supportcharge recombination comprises a matrix and one or more nanostructures,where the one or more nanostructures each comprise a core and at leastone shell. The core comprises a first semiconducting material having aconduction band and a valence band, and the shell comprises a secondsemiconducting material having a conduction band and a valence band. Thefirst and second materials have a type I band offset. Examples of Type Iband offsets are shown in FIG. 8 Panels B and D (as depicted, the bandsare symmetrically offset, but this need not be the case). In oneembodiment, the conduction band 89 of the first material is lower thanthe conduction band 91 of the second material, and the valence band 90of the first material is higher than the valence band 92 of the secondmaterial. (Electron-hole pairs created in the core are thus confined tothe core, where they can recombine.) In an alternative embodiment, theconduction band 93 of the first material is higher than the conductionband 95 of the second material, and the valence band 94 of the firstmaterial is lower than the valence band 96 of the second material.

The first material comprising the core can comprise e.g., a materialcomprising a first element selected from group 2 or from group 12 of theperiodic table and a second element selected from group 16 (e.g., ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS,CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and like materials); amaterial comprising a first element selected from group 13 and a secondelement selected from group 15 (e.g., GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, and like materials); a material comprising a group 14element (Ge, Si, and like materials); a material such as PbS, PbSe,PbTe, AlS, AlP, and AlSb; or an alloy or a mixture thereof. The secondmaterial comprising the shell can comprise e.g. any of the abovesemiconducting materials having an appropriate band offset with respectto the first material. (When a second or higher shell is present, it cancomprise essentially any convenient material that is different and/ordistinguishable from the second material.)

The matrix can e.g. comprise at least one polymer, comprise at least oneglass, or be a small molecule or molecular matrix. The matrix canconduct both electrons and holes, conduct substantially only holes,conduct substantially only electrons, be semiconducting, or besubstantially nonconductive. Example matrices include those mentionedand/or referenced herein. The nanostructures can comprise e.g.,nanocrystals, nanowires, branched nanowires (e.g., nanotetrapods,tripods, or bipods), nanoparticles, or other nanostructures.

In another class of embodiments, a composite material that can supportcharge recombination comprises one or more nanostructures comprising afirst semiconducting material having a conduction band and a valenceband and a matrix comprising a second semiconducting material having aconduction band and a valence band. The first and second materials havea type I band offset. In one embodiment, the conduction band of thefirst material is lower than the conduction band of the second material,and the valence band of the first material is higher than the valenceband of the second material. In an alternative embodiment, theconduction band of the first material is higher than the conduction bandof the second material, and the valence band of the first material islower than the valence band of the second material.

In some embodiments, the nanostructures are heterostructures. Forexample, in one embodiment, each nanostructure comprises a core and atleast one shell, and the core comprises the first material. In anotherembodiment, each nanostructure comprises a core and at least one shell,and the shell comprises the first material. In other embodiments, thenanostructures are not heterostructures, e.g., each nanostructure cancomprise substantially a single material, the first material.

The first material can comprise e.g., a material comprising a firstelement selected from group 2 or from group 12 of the periodic table anda second element selected from group 16 (e.g., ZnS, ZnSe, ZnTe, CdS,CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS,SrSe, SrTe, BaS, BaSe, BaTe, and like materials); a material comprisinga first element selected from group 13 and a second element selectedfrom group 15 (e.g., GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, andlike materials); a material comprising a group 14 element (Ge, Si, andlike materials); a material such as PbS, PbSe, PbTe, AlS, AlP, and AlSb;or an alloy or a mixture thereof.

The matrix can comprise e.g. at least one polymer, comprise at least oneglass, or be a small molecule or molecular matrix. The secondsemiconducting material comprising the matrix can be essentially anyknown in the art having an appropriate band offset from the firstmaterial. Examples include those matrices mentioned and/or referencedherein (see e.g. T. A. Skatherin (ed.) Handbook of Conducting PolymersI), and include but are not limited to poly(3-hexylthiophene) (P3HT),poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV),poly(phenylene vinylene) (PPV), and polyaniline. The nanostructures cancomprise e.g., nanocrystals, nanowires, branched nanowires (e.g.,nanotetrapods, tripods, or bipods), nanoparticles, or othernanostructures.

In one class of embodiments, a composite material that can supportcharge separation comprises a matrix and one or more nanostructures,where the one or more nanostructures each comprise a core and at leastone shell. The core comprises a first semiconducting material having aconduction band and a valence band, and the shell comprises a secondsemiconducting material having a conduction band and a valence band. Thefirst and second materials have a type II band offset. (In such acomposite, when an electron-hole pair is created in the core of ananostructure, one partner (either the electron or the hole) tends totravel, e.g. through the surrounding matrix, while the other partner isconfined to the core.) Examples of Type II band offsets are shown inFIG. 8 Panels A and C (as depicted, the bands are symmetrically offset,but this need not be the case). In one embodiment, the conduction band81 of the first material is lower than the conduction band 83 of thesecond material, and the valence band 82 of the first material is lowerthan the valence band 84 of the second material. In an alternativeembodiment, the conduction band 85 of the first material is higher thanthe conduction band 87 of the second material, and the valence band 86of the first material is higher than the valence band 88 of the secondmaterial.

The first material comprising the core can comprise e.g., a materialcomprising a first element selected from group 2 or from group 12 of theperiodic table and a second element selected from group 16 (e.g., ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS,CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and like materials); amaterial comprising a first element selected from group 13 and a secondelement selected from group 15 (e.g., GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, and like materials); a material comprising a group 14element (Ge, Si, and like materials); a material such as PbS, PbSe,PbTe, AlS, AlP, and AlSb; or an alloy or a mixture thereof. The secondmaterial comprising the shell can comprise e.g. any of the abovesemiconducting materials having an appropriate band offset with respectto the first material. (When a second or higher shell is present, it cancomprise essentially any convenient material that is different and/ordistinguishable from the second material.)

The matrix can e.g. comprise at least one polymer, comprise at least oneglass, or be a small molecule or molecular matrix. The matrix canconduct both electrons and holes, conduct substantially only holes,conduct substantially only electrons, be semiconducting, or besubstantially nonconductive. Example matrices include those mentionedand/or referenced herein. The nanostructures can comprise e.g.,nanocrystals, nanowires, branched nanowires (e.g., nanotetrapods,tripods, or bipods), nanoparticles, or other nanostructures.

In another class of embodiments, a composite material that can supportcharge separation comprises one or more nanostructures comprising afirst semiconducting material having a conduction band and a valenceband and a matrix comprising a second semiconducting material having aconduction band and a valence band. The first and second materials havea type II band offset. In one embodiment, the conduction band of thefirst material is lower than the conduction band of the second material,and the valence band of the first material is lower than the valenceband of the second material. In an alternative embodiment, theconduction band of the first material is higher than the conduction bandof the second material, and the valence band of the first material ishigher than the valence band of the second material.

In some embodiments, the nanostructures are heterostructures. Forexample, in one embodiment, each nanostructure comprises a core and atleast one shell, and the core comprises the first material. In anotherembodiment, each nanostructure comprises a core and at least one shell,and the shell comprises the first material. In one specific embodiment,each nanostructure comprises a core comprising a third semiconductingmaterial having a conduction band and a valence band and at least oneshell comprising the first material; the third and first materials (thecore and the shell) have a type II band offset, and the first and secondmaterials (the shell and the matrix) have a type II band offset. Inother embodiments, the nanostructures are not heterostructures, e.g.,each nanostructure can comprise substantially a single material, thefirst material.

The first material can comprise e.g., a material comprising a firstelement selected from group 2 or from group 12 of the periodic table anda second element selected from group 16 (e.g., ZnS, ZnSe, ZnTe, CdS,CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS,SrSe, SrTe, BaS, BaSe, BaTe, and like materials); a material comprisinga first element selected from group 13 and a second element selectedfrom group 15 (e.g., GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, andlike materials); a material comprising a group 14 element (Ge, Si, andlike materials); a material such as PbS, PbSe, PbTe, AlS, AlP, and AlSb;or an alloy or a mixture thereof.

The matrix can comprise e.g. at least one polymer, comprise at least oneglass, or be a small molecule or molecular matrix. The secondsemiconducting material comprising the matrix can be essentially anyknown in the art having an appropriate band offset from the firstmaterial. Examples include those matrices mentioned and/or referencedherein (see e.g. T. A. Skatherin (ed.) Handbook of Conducting PolymersI), and include but are not limited to poly(3-hexylthiophene) (P3HT),poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV),poly(phenylene vinylene) (PPV), and polyaniline. The nanostructures cancomprise e.g., nanocrystals, nanowires, branched nanowires (e.g.,nanotetrapods, tripods, or bipods), nanoparticles, or othernanostructures.

In any of the above embodiments, the nanostructures are optionallysubstantially nonrandomly oriented in the matrix. For example, acomposite material supporting charge recombination or charge separationcan comprise a plurality of nanowires that have their long axes morenearly perpendicular than parallel to a first plane (e.g., a surface ofa film comprising the composite material); e.g., each nanowire can besubstantially perpendicular to the first plane. Similarly, the nanowirescan have their long axes more nearly parallel than perpendicular to thefirst plane; e.g., each nanowire can be substantially parallel to thefirst plane.

Surface Ligands

Nanostructures (e.g., nanowires, branched nanowires, nanocrystals, ornanoparticles) can comprise one or more surface ligands. Suchsurface-bound molecules include, for example, surfactant molecules.Surfactants are used, for example, to control the size and/or shape ofthe nanostructures during their growth process. See e.g. Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61. Surfaceligands can be incorporated during the growth of the nanostructures, orcan be incorporated after the nanostructures are fully formed (forexample, through surfactant exchange, in which the nanostructures aretaken from their growth solution and refluxed several times in asolution containing the desired ligand). See e.g., U.S. patentapplication Ser. No. 60/389,029 (filed Jun. 13, 2002) by Empedoclesentitled “Nanotechnology enabled optoelectronics.”

One class of embodiments provides a composite containing nanostructuresand a matrix having enhanced affinity for each other, thereby increasingsolubility of the nanostructures in and/or assisting dispersal of thenanostructures throughout the matrix.

One embodiment provides a composite material comprising a plurality ofnanostructures and a small molecule or molecular matrix or matrixcomprising at least one polymer, where the polymer or the constituentsof the small molecule or molecular matrix have an affinity for at leasta portion of the surface of the nanostructures. Another embodimentprovides a composite material comprising a small molecule or molecularmatrix or matrix comprising at least one polymer and a plurality ofnanostructures, where the nanostructures comprise one or more surfaceligands for which the polymer or the constituents of the small moleculeor molecular matrix have an affinity.

In a preferred embodiment, the surface ligands on the nanostructurescomprise at least one molecule found in the small molecule or molecularmatrix or a derivative thereof or at least one monomer found in the atleast one polymer or a derivative thereof. The molecules comprising thesmall molecule or molecular matrix can be any of those known in the art,e.g., molecular organics such as those used in OLEDs;N,N′-diphenyl-N,N′-bis (3-methylphenyl)-(1,1′biphenyl)-4,4′-diamine)(TPD); (3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole)(TAZ); tris-(8-hydroxyquinoline) aluminum (Alq₃); benzoic acid; phthalicacid; benzoin; hydroxyphenol; nitrophenol; chlorophenol; chloroaniline;or chlorobenzoamide. Similarly, the monomer can be essentially anymonomer, e.g., a monomeric unit of an organic, an inorganic, or anorganometallic polymer (e.g., a polysiloxane, a polycarbonessiloxane, apolyphosphazene, a ferrocene polymer, a platinum polymer, a palladiumpolymer, a thermoplastic polymer (e.g., a polyolefin, a polyester, apolysilicone, a polyacrylonitrile resin, a polystyrene resin, polyvinylchloride, polyvinylidene chloride, polyvinyl acetate, or afluoroplastic), a thermosetting polymer (e.g., a phenolic resin, a urearesin, a melamine resin, an epoxy resin, a polyurethane resin, anengineering plastic, a polyamide, a polyacrylate resin, a polyketone, apolyimide, a polysulfone, a polycarbonate, or a polyacetal), a liquidcrystal polymer, including a main chain liquid crystal polymer (e.g.,poly(hydroxynapthoic acid) or a side chain liquid crystal polymer (e.g.,poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>, or a conductingpolymer (e.g., poly(3-hexylthiophene) (P3HT), poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV), poly(phenylenevinylene) (PPV), or polyaniline). See e.g., Dietrich Demus, John W.Goodby, George W. Gray, Hans W. Spiess, and Volkmar Vill (1998) Handbookof Liquid Crystals, Handbook of Liquid Crystals: Four Volume Set, JohnWiley and Sons, Inc.; Johannes Brandrup (1999) Polymer Handbook, JohnWiley and Sons, Inc.; Charles A. Harper (2002) Handbook of Plastics,Elastomers, and Composites, 4^(th) edition, McGraw-Hill; T. A. Skatherin(ed.) Handbook of Conducting Polymers I; and Ronald Archer Inorganic andOrganometallic Polymers for other examples.) For example, a typicalsurface ligand can comprise a derivative of one of the above monomers ormolecules, where the derivative comprises at least one group thatattaches to the surface of the nanostructure (e.g., an amine, aphosphine, a phosphine oxide, a phosphonate, a phosphonite, a phosphinicacid, a phosphonic acid, a thiol, an alcohol, or an amine oxide). Asanother example, the derivative can comprise a group with an affinityfor a second surface ligand attached to the surface of thenanostructure.

The surface ligands can each comprise e.g. at least one functional groupto bind to the surface of the nanostructures, e.g., an amine, aphosphine, a phosphine oxide, a phosphonate, a phosphonite, a phosphinicacid, a phosphonic acid, a thiol, an alcohol, or an amine oxide.

High Dielectric Composites and Compositions

One general class of embodiments provides nanocomposites that can have ahigh dielectric constant as well as compositions for making suchnanocomposites. The composite materials comprise ferroelectric nanowiresor nanoparticles. The composites have a number of uses. For example,they can be formed into thin insulating films, such as those printed oncircuit boards. In addition, the nanocomposites can be used in datastorage media, where information is encoded by flipping theferroelectric state of individual nanowires or nanoparticles to writebits.

Nanocomposites

The composite materials of this embodiment comprise one or moreferroelectric nanowires or one or more ferroelectric nanoparticles and asmall molecule or molecular matrix or a matrix comprising one or morepolymers. The nanowires or nanoparticles can comprise essentially anyconvenient ferroelectric or ferroelectric ceramic material. For example,the nanowires or nanoparticles can comprise a perovskite-type material(including but not limited to BaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃,LiTiO₃, LiTaO₃, or LiNbO₃, or a material derived from these, forexample, Ba_((1-x))Ca_(x)TiO₃ where x is between 0 and 1, orPbTi_((1-x))Zr_(x)O₃ where x is between 0 and 1); a KDP-type material(e.g., KH₂PO₄, KD₂PO₄, RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄, or GeTe, or amaterial derived from these); a TGS-type material (e.g., tri-glycinesulfate, tri-glycine selenate, or a material derived from these); or amixture thereof

The diameter of the nanowires can be varied. For example, the averagediameter of the nanowires can be preferably between about 2 nm and about100 nm, or more preferably between about 2 nm and about 5 nm or betweenabout 10 nm and about 50 nm. The length of the nanowires can also bevaried. In certain embodiments, the nanowires have an aspect ratiobetween about 1.5 and about 10,000 (e.g., between about 1.5 and about10, between about 10 and about 20, between about 20 and about 50,between about 50 and about 10,000, or between about 100 and about10,000).

The size of the nanoparticles can also be varied. In a preferredembodiment, the one or more ferroelectric nanoparticles have an averagediameter less than about 200 nm (e.g., less than about 100 nm, or lessthan about 50 nm). In another embodiment, the nanoparticles are roughlyspherical, having an aspect ratio between about 0.9 and about 1.2.

A wide variety of polymers are known to those of skill in art and can beused in this invention. For example, the composite can comprise aninorganic polymer (e.g. a polysiloxane, a polycarbonessiloxane (acopolymer of siloxane and carborane), or a polyphosphazene) or anorganometallic polymer (e.g., a ferrocene polymer, a platinum polymer,or a palladium polymer). For other examples of inorganic andorganometallic polymers see e.g. Ronald Archer Inorganic andOrganometallic Polymers. As another example, the composite can comprisean organic polymer, e.g., a thermoplastic polymer (e.g., a polyolefin, apolyester, a polysilicone, a polyacrylonitrile resin, a polystyreneresin, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate,or a fluoroplastic), a thermosetting polymer (e.g., a phenolic resin, aurea resin, a melamine resin, an epoxy resin, a polyurethane resin, anengineering plastic, a polyamide, a polyacrylate resin, a polyketone, apolyimide, a polysulfone, a polycarbonate, or a polyacetal), or a liquidcrystal polymer, including a main chain liquid crystal polymer (e.g.,poly(hydroxynapthoic acid) or a side chain liquid crystal polymer (e.g.,poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>). For otherexamples see e.g., Dietrich Demus, John W. Goodby, George W. Gray, HansW. Spiess, and Volkmar Vill (1998) Handbook of Liquid Crystals, Handbookof Liquid Crystals: Four Volume Set, John Wiley and Sons, Inc.; JohannesBrandrup (1999) Polymer Handbook, John Wiley and Sons, Inc.; and CharlesA. Harper (2002) Handbook of Plastics, Elastomers, and Composites,4^(th) edition, McGraw-Hill; and T. A. Skatherin (ed.) Handbook ofConducting Polymers I. Alternatively, the composite can comprise a smallmolecule or molecular matrix. A variety of such matrices are known inthe art, comprising e.g., molecular organics such as those used inOLEDs; N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′biphenyl)-4,4′-diamine) (TPD);(3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole) (TAZ);tris-(8-hydroxyquinoline) aluminum (Alq₃); benzoic acid; phthalic acid;benzoin; hydroxyphenol; nitrophenol; chlorophenol; chloroaniline; orchlorobenzoamide. Preferred small molecules have at least one polarfunctional group and have a high enough melting point that they aresolid at room temperature.

The polymeric or small molecule or molecular matrix can optionallycomprise one or more additives. An additive can be used, for example, toassist in forming the composite. Examples include a catalyst used tocross-link a polymeric matrix or a surfactant used to disperse nanowiresor nanoparticles throughout the matrix. As another example, an additivecan be used to affect one or more physical properties of the matrixand/or the composite, e.g., the additive can be a plasticizer, astrengthening fiber, or an antioxidant.

The dielectric constant of the composite material can be controlled byvarying the amount of nanowires or nanoparticles added to the composite.Typically, the composite comprises nanowires or nanoparticles in anamount greater than 0% and less than about 90% by volume (e.g., greaterthan about 0% to less than about 40% by volume, greater than about 0.5%to less than about 25% by volume, greater than about 1% to less thanabout 15% by volume, or greater than about 15% to about 40% by volume).In a preferred embodiment, the ferroelectric nanowires or nanoparticlesare included in sufficient quantity that the composite has a dielectricconstant of at least about 2, more preferably at least about 5, or mostpreferably at least about 10.

The composite can be formed into a film (e.g., an insulating filmapplied over a microelectronic device), a sheet, or other useful shape.Similarly, the composite material can be applied to a substrate (e.g., acircuit board). The substrate can comprise, for example, silicon, glass,an oxide, a metal, or a plastic.

The composite material can also be incorporated into compositions. Oneembodiment provides a composition comprising particles of the compositematerial, at least one solvent, and at least one glue agent. The glueagent can be any substance capable of sticking the particles of thecomposite material together; for example, a polymer or a cross-linker.The particles of the composite material typically have an averagediameter between about 20 nm and about 20 micrometers (e.g., betweenabout 100 nm and about 20 micrometers), but can be smaller or larger.The composition can be used to form a nanocomposite film, for example,after application to a substrate. The at least one solvent can beessentially any convenient solvent, for example, water or an organicsolvent (e.g., an alcohol, a ketone, an acetate, an amine, a diol, aglycol, or a glycol ether). The solvent concentration can be adjusted toadjust the viscosity of the composition to render it suitable forapplication to essentially any desired substrate by any convenientmethod (e.g., inkjet printing, screen printing, brushing, or spraying).The composition can optionally comprise one or more additives, forexample, a surfactant to assist in dispersal of the particles of thecomposite material or a humectant. The particles of the compositematerial can be formed according to methods known in the art, e.g., bygrinding a solid composite, or e.g. by spray drying a suspension of acomposition as described below.

Compositions

Two classes of embodiments provide compositions that can be used to formnanocomposites. For example, the compositions can be applied to asubstrate to form a nanocomposite film on the substrate.

One class of embodiments provides a composition that comprises one ormore ferroelectric nanowires or one or more ferroelectric nanoparticles,at least one solvent, and one or more polymers. The nanowires ornanoparticles can comprise essentially any convenient ferroelectric orferroelectric ceramic material. For example, the nanowires ornanoparticles can comprise a perovskite-type material (including but notlimited to BaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃, LiTiO₃, LiTaO₃, orLiNbO₃, or a material derived from these, for example,Ba_((1-x))Ca_(x)TiO₃ where x is between 0 and 1, or PbTi_((1-x))Zr_(x)O₃where x is between 0 and 1); a KDP-type material (e.g., KH₂PO₄, KD₂PO₄,RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄, or GeTe, or a material derived from these);a TGS-type material (e.g., tri-glycine sulfate, tri-glycine selenate, ora material derived from these); or a mixture thereof.

The diameter of the nanowires can be varied. For example, the averagediameter of the nanowires can be preferably between about 2 nm and about100 nm, or more preferably between about 2 nm and about 5 nm or betweenabout 10 nm and about 50 nm. The length of the nanowires can also bevaried. In certain embodiments, the nanowires have an aspect ratiobetween about 1.5 and about 10,000 (e.g., between about 1.5 and about10, between about 10 and about 20, between about 20 and about 50,between about 50 and about 10,000, or between about 100 and about10,000).

The size and shape of the nanoparticles can also be varied. For example,the one or more ferroelectric nanoparticles can have an average diameterless than about 200 nm (e.g., less than about 100 nm, or less than about50 nm), and/or the nanoparticles can be roughly spherical, having anaspect ratio between about 0.9 and about 1.2.

A wide variety of polymers are known to those of skill in the art andcan be used in this invention. For example, the composition can comprisean inorganic polymer (e.g. a polysiloxane, a polycarbonessiloxane (acopolymer of siloxane and carborane), or a polyphosphazene) or anorganometallic polymer (e.g., a ferrocene polymer, a platinum polymer,or a palladium polymer); see also e.g. Ronald Archer Inorganic andOrganometallic Polymers. As another example, the composite can comprisean organic polymer, e.g., a thermoplastic polymer (e.g., a polyolefin, apolyester, a polysilicone, a polyacrylonitrile resin, a polystyreneresin, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate,or a fluoroplastic), a thermosetting polymer (e.g., a phenolic resin, aurea resin, a melamine resin, an epoxy resin, a polyurethane resin, anengineering plastic, a polyamide, a polyacrylate resin, a polyketone, apolyimide, a polysulfone, a polycarbonate, or a polyacetal), or a liquidcrystal polymer, including a main chain liquid crystal polymer (e.g.,poly(hydroxynapthoic acid) or a side chain liquid crystal polymer (e.g.,poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>). For otherexamples see e.g., Dietrich Demus, John W. Goodby, George W. Gray, HansW. Spiess, and Volkmar Vill (1998) Handbook of Liquid Crystals Handbookof Liquid Crystals: Four Volume Set, John Wiley and Sons, Inc.; JohannesBrandrup (1999) Polymer Handbook, John Wiley and Sons, Inc.; and CharlesA. Harper (2002) Handbook of Plastics, Elastomers, and Composites,4^(th) edition, McGraw-Hill; and T. A. Skatherin (ed.) Handbook ofConducting Polymers I.

The one or more polymers can be provided in various forms. For example,in one embodiment, the polymer(s) are soluble in the at least onesolvent (e.g., polyacrylic acid in water, or PVDF in acetone). Inanother embodiment, the polymer(s) comprise oligomers that are solublein the solvent. In this embodiment, the composition can further compriseat least one cross-linking agent (e.g., a cross-linker and/or acatalyst, for cross-linking the oligomers after the composition has beenapplied to a surface). For example, the composition could comprisedimethylsiloxane oligomers, siloxane cross-linkers (which are alsooligomers), and a catalyst, such that application and curing of thecomposition results in a PDMS matrix comprising ferroelectric nanowiresor nanoparticles. In yet another embodiment, the polymer(s) compriseemulsion polymerized polymer particles that are dispersed in the atleast one solvent. In this embodiment, the composition can furthercomprise at least one glue agent, for example, a polymer orcross-linking agent. Emulsion polymerized particles of various polymersare commercially available; for example, emulsions of polyolefins andpolyacrylates are available from Air Products and Chemicals, Inc.(www.airproducts.com). The size of available polymer particles istypically in the range of about 10 nm to about 200 nm.

The composition can optionally further comprise at least one surfactant(e.g., a cationic, anionic, or nonionic surfactant) and/or at least onehumectant (e.g., a glycol, a diol, a sulfoxide, a sulfone, an amide, oran alcohol).

The at least one solvent can be essentially any convenient solvent, forexample, water or an organic solvent (e.g., an alcohol, a ketone, anacetate, an amine, a diol, a glycol, or a glycol ether). The solventconcentration can be adjusted to control the consistency of thecomposition. For example, the composition can be a liquid suitable foruse as an inkjet printing ink or can be a paste suitable for use as ascreen printing ink. For example, the composition can have a consistencythat makes it suitable for applying it to a surface by brushing orspraying.

The composition can be used to form a film. For example, afterapplication to a substrate, the composition can form a nanocompositefilm. The composition can be applied to essentially any desiredsubstrate, for example a substrate that comprises silicon, glass, anoxide, a metal, or a plastic.

Another class of embodiments provides a composite that comprises one ormore ferroelectric nanowires or one or more ferroelectric nanoparticlesand at least one monomeric precursor of at least one polymer. Thenanowires or nanoparticles can comprise essentially any convenientferroelectric or ferroelectric ceramic material. For example, thenanowires or nanoparticles can comprise a perovskite-type material(including but not limited to BaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃,LiTiO₃, LiTaO₃, or LiNbO₃, or a material derived from these, forexample, Ba_((1-x))Ca_(x)TiO₃ where x is between 0 and 1, orPbTi_((1-x))Zr_(x)O₃ where x is between 0 and 1); a KDP-type material(e.g., KH₂PO₄, KD₂PO₄, RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄, or GeTe, or amaterial derived from these); a TGS-type material (e.g., tri-glycinesulfate, tri-glycine selenate, or a material derived from these); or amixture thereof.

The diameter of the nanowires can be varied. For example, the averagediameter of the nanowires can be preferably between about 2 nm and about100 nm, or more preferably between about 2 nm and about 5 nm or betweenabout 10 nm and about 50 nm. The length of the nanowires can also bevaried. In certain embodiments, the nanowires have an aspect ratiobetween about 1.5 and about 10,000 (e.g., between about 1.5 and about10, between about 10 and about 20, between about 20 and about 50,between about 50 and about 10,000, or between about 100 and about10,000).

The size and shape of the nanoparticles can also be varied. For example,the one or more ferroelectric nanoparticles can have an average diameterless than about 200 nm (e.g., less than about 100 nm, or less than about50 nm), and/or the nanoparticles can be roughly spherical, having anaspect ratio between about 0.9 and about 1.2.

The at least one monomeric precursor can comprise a monomer ofessentially any polymer known to one of skill, e.g., the organic andinorganic polymers discussed herein. The polymer is preferablynonconductive. Some specific examples include e.g., styrene, acrylate,acrylonitrile, acrylamide, acrylic acid, and vinyl acetate.

The composition can further comprise at least one catalyst to catalyzepolymerization of the monomers and/or at least one solvent whoseconcentration can be adjusted to adjust the consistency of thecomposition to suit various application techniques (e.g., water or anorganic solvent). The composition can also further comprise at least onesurfactant and/or at least one humectant.

The composition can be used to form a film. For example, afterapplication to a substrate and polymerization, the composition can forma nanocomposite film. The composition can be applied to essentially anydesired substrate, for example a substrate that comprises silicon,glass, an oxide, a metal, or a plastic.

Methods for Making Composites and Compositions

One aspect of the invention provides methods for making the compositematerials and compositions described herein. All the methods involvecombining preformed nanostructures with one or more other materials toform the composite or composition.

One method for making a composite material comprises preparing one ormore ferroelectric nanowires or nanoparticles and combining thepreformed nanowires or nanoparticles with at least one polymer or theprecursors or constituents of a small molecule or molecular matrix.Components of the composite can be varied as described above. Forexample, the ferroelectric nanowires or nanoparticles can compriseessentially any convenient ferroelectric or ferroelectric ceramicmaterial, e.g., a perovskite-type material (including but not limited toBaTiO₃, SrTiO₃, CaTiO₃, KNbO₃, PbTiO₃, LiTiO₃, LiTaO₃, or LiNbO₃, or amaterial derived from these, for example, Ba_((1-x))Ca_(x)TiO₃ where xis between 0 and 1, or PbTi_((1-x))Zr_(x)O₃ where x is between 0 and 1);a KDP-type material (e.g., KH₂PO₄, KD₂PO₄, RbH₂PO₄, RbH₂AsO₄, KH₂AsO₄,or GeTe, or a material derived from these); a TGS-type material (e.g.,tri-glycine sulfate, tri-glycine selenate, or a material derived fromthese); or a mixture thereof Typically, the composite comprisesnanowires or nanoparticles in an amount greater than 0% and less thanabout 90% by volume (e.g., greater than about 0% to less than about 40%by volume, greater than about 0.5% to less than about 25% by volume, orgreater than 1% to less than about 15% by volume). In a preferredembodiment, the ferroelectric nanowires or nanoparticles are included insufficient quantity that the composite has a dielectric constant of atleast about 2, more preferably at least about 5, or most preferably atleast about 10. The at least one polymer can comprise e.g. an inorganicpolymer (e.g., a polysiloxane, a polycarbonessiloxane, or apolyphosphazene), an organometallic polymer (e.g., a ferrocene polymer,a platinum polymer, or a palladium polymer), or an organic polymer,e.g., a thermoplastic polymer (e.g., a polyolefin, a polyester, apolysilicone, a polyacrylonitrile resin, a polystyrene resin, polyvinylchloride, polyvinylidene chloride, polyvinyl acetate, or afluoroplastic), a thermosetting polymer (e.g., a phenolic resin, a urearesin, a melamine resin, an epoxy resin, a polyurethane resin, anengineering plastic, a polyamide, a polyacrylate resin, a polyketone, apolyimide, a polysulfone, a polycarbonate, or a polyacetal), or a liquidcrystal polymer, including a main chain liquid crystal polymer (e.g.,poly(hydroxynapthoic acid) or a side chain liquid crystal polymer (e.g.,poly <n-((4′(4″-cyanphenyl)phenoxy)alkyl)vinyl ether>). The nanowires ornanoparticles and a polymer can be combined with one or more additives,for example, a surfactant, a catalyst, a plasticizer, an antioxidant, ora strengthening fiber.

Another method for making a composite material comprises preparing oneor more branched nanowires or one or more inorganic semiconducting orferroelectric nanowires having an aspect ratio greater than about 10(e.g., greater than about 15, greater than about 20, or greater thanabout 50). The preformed branched nanowires or inorganic nanowires (or acombination thereof) are then combined with at least one organic polymeror inorganic glass or the precursors or constituents of a small moleculeor molecular matrix to produce a nanocomposite. Components of thecomposite can be varied as described above.

Another method for making a composite material comprises preparing oneor more nanostructures and incorporating the preformed nanostructuresinto a polymeric matrix comprising polysiloxane (e.g.,polydimethylsiloxane). Components of the composite can be varied asdescribed above.

A method for making a composition comprises preparing particles of acomposite material, where the composite comprises ferroelectricnanowires or nanoparticles and a small molecule or molecular matrix ormatrix comprising one or more polymers, and combining the compositeparticles with at least one solvent and at least one glue agent.Components of the composition can be varied as described above.

Another method of making a composition comprises preparing one or moreferroelectric nanowires or nanoparticles and combining them with atleast one solvent and one or more polymers. The one or more polymers cantake various forms. For example, the polymer(s) can be soluble in thesolvent, comprise oligomers soluble in the solvent, or comprise emulsionpolymerized particles capable of being suspended in the solvent.Components of the composition can be varied as described above.

Another method of making a composition comprises preparing one or moreferroelectric nanowires or nanoparticles and combining them with atleast one monomeric precursor of at least one polymer. Components of thecomposition can be varied as described above.

General Considerations

Composite materials comprising nanostructures can be made according tomethods known in the art. See, e.g., Dufresne et al. (1996) “Newnanocomposite materials: Microcrystalline starch reinforcedthermoplastic” Macromolecules 29, 7624-7626; and Angles et al. (2001)“Plasticized starch/tunicin whiskers nanocomposite materials. 2.Mechanical behavior” Macromolecules 34, 2921-2931. For example, ananocomposite can be formed by dispersing nanostructures in a solutioncomprising components of the matrix (e.g., a small molecule, a monomer,an oligomer, a polymer dissolved in a solvent, a polymer emulsion, acopolymer, or a combination thereof) and optionally an additive (e.g., asurfactant, a catalyst, a strengthening fiber, a plasticizer, or anantioxidant) by agitation (e.g., ultrasonication or mechanical stirring)and then if necessary removing any excess solvent. The nanocomposite cane.g. be applied to a surface (e.g., by spin casting) prior to removal ofany excess solvent. As another example, small molecule matrices can becreated using a combination of e.g. evaporative, spray coating, orspin-coating processes. The evaporation of small molecules andcondensation onto a substrate allows for finely controlled depositionand the design of complex layered structures.

Synthesis of Nanostructures

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape control of CdSenanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals”; U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers”; U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same”; and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process”; U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys”;U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques”; and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. patent application Ser. No.60/370,095 (Apr. 2, 2002) to Empedocles entitled “Nanowireheterostructures for encoding information.” Similar approaches can beapplied to growth of other heterostructures.

In certain embodiments, the collection or population of nanostructuresis substantially monodisperse in size and/or shape. See e.g., US patentapplication 20020071952 by Bawendi et al entitled “Preparation ofnanocrystallites.”

Nanostructures optionally comprise additional elements to enhance theirfunction. For example, nanostructures (e.g., crystalline nanostructures)can optionally be dye sensitized to increase light absorption and/orcharge injection into the nanostructure. Examples of such dyes includethose described in U.S. Pat. No. 6,245,988 and published PCT ApplicationNos. WO 94/04497 and 95/29924, where ruthenium-based dyes are providedto enhance light absorption and charge injection.

Controlling Nanostructure Emission Spectra

As indicated previously, nanostructures can be fabricated fromfluorescent materials (among many other materials). The composition,size, and shape of such a nanostructure affects its emission spectrum(the wavelength(s) of light emitted by the nanostructure). The emissionspectrum of a nanostructure can thus be controlled by controlling itscomposition, size, and shape.

Exemplary semiconducting materials that at certain size ranges emitenergy in the visible range include, but are not limited to, CdS, CdSe,CdTe, ZnSe, ZnTe, GaP, and GaAs. Semiconductor materials that can emitenergy in the near-infrared range include, for example, InP, InAs, InSb,PbS, and PbSe. Semiconducting materials that can emit energy in the blueto near-ultraviolet range include, for example, ZnS and GaN.

The color of light produced by a particular size, size distribution,shape, and/or composition of a nanostructure can be readily calculatedor measured by methods known to those skilled in the art. See, forexample, Murray et al. (1993) J. Am. Chem. Soc. 115, 8706, and Li et al.(2001) “Band gap variation of size- and shape-controlled colloidal CdSequantum rods” Nanoletters 1, 349-351.

Fluorescent nanostructures can each be a single material or aheterostructure, in which case both (or all) of the multiple materialscomprising the nanostructure can affect emission. For example, a shellcan be chosen to enhance the efficiency of emission by the core, bymitigating adverse effects caused by defects at the surface of thenanostructure that can result in traps for electrons or holes thatdegrade the electrical and optical properties of the material. Aninsulating layer at the surface of the nanostructure can provide anatomically abrupt jump in the chemical potential at the interface thateliminates energy states that can serve as traps for the electrons andholes. This results in higher efficiency in the luminescent process.

Suitable materials for a fluorescence efficiency-enhancing shell includee.g. materials having a higher band gap energy than the material formingthe nanostructure's core. In addition to having a band gap energygreater than that of the core material, suitable materials for the shellcan have e.g. good conduction and valence band offset with respect tothe core material. That is, the conduction band is preferably higher andthe valence band is preferably lower than those of the core material.For cores that emit energy in the visible range (e.g., CdS, CdSe, CdTe,ZnSe, ZnTe, GaP, GaAs) or near-infrared (e.g., InP, InAs, InSb, PbS,PbSe), a material that has a band gap energy e.g. in the ultravioletrange can be used (e.g., ZnS, GaN, and magnesium chalcogenides, e.g.,MgS, MgSe, and MgTe). For a core that emits in the near-infrared,materials having a band gap energy e.g. in the visible range (e.g. CdSor CdSe) can also be used.

Similar considerations can apply when choosing materials for other typesof heterostructures (e.g., in a nanowire heterostructure where thedifferent materials are distributed at different locations along thelong axis of the nanowire). For example, higher band gap energy regionsbounding a lower band gap energy region within such a nanowireheterostructure can improve fluorescence emission intensity, just as ashell or shells can improve emission by the core in a core-shellheterostructure.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. For example, all the techniques, methods, compositions, andapparatus described above can be used in various combinations. Allpublications, patents, patent applications, and/or other documents citedin this application are incorporated by reference in their entirety forall purposes to the same extent as if each individual publication,patent, patent application, and/or other document were individuallyindicated to be incorporated by reference for all purposes.

1. A waveguide, comprising: a cladding; and a first core, the corehaving a first surface and a second surface that are substantiallyparallel to each other, the core having a higher index of refractionthan the cladding; and a first layer distributed within the first corecomprising one or more nanowires or one or more branched nanowires and amatrix, and the core being in contact with the cladding over at least amajority of the first and second surfaces of the core.
 2. A waveguide asin claim 1, wherein the one or more branched nanowires comprise one ormore nanotetrapods.
 3. A waveguide as in claim 1, wherein the claddingis air.
 4. A waveguide as in claim 1, wherein the waveguide is a flatsheet.
 5. A waveguide as in claim 1, wherein the core has an index ofrefraction between about 1.35 and about
 4. 6. A waveguide as in claim 1,wherein the matrix is substantially nonabsorbing and nonscattering withrespect to light at wavelengths greater than about 300 nm.
 7. Awaveguide as in claim 1, wherein the matrix comprises a glass, apolymer, a small molecule or molecular matrix, a liquid, a crystal, or apolycrystal.
 8. A waveguide as in claim 1, wherein the one or morenanowires have an average diameter between about 2 nm and about 100 nmor between about 2 nm and about 20 nm.
 9. A waveguide as in claim 1,wherein the one or more nanowires have an aspect ratio between about 1.5and about 100, or between about 5 and about
 30. 10. A waveguide as inclaim 1, wherein the waveguide comprises a plurality of nanowires.
 11. Awaveguide as in claim 10, wherein the orientation of the nanowires issubstantially nonrandom, with a vector average of the nanowires'orientations having a nonzero component perpendicular to the firstsurface of the core.
 12. A waveguide as in claim 11, wherein at least1%, at least 10%, or at least 50% of the total nanowires within the coreare substantially nonrandomly oriented.
 13. A waveguide as in claim 11,wherein a majority of the nanowires each has a long axis oriented morenearly perpendicular than parallel to the first surface of the core. 14.A waveguide as in claim 11, wherein the plurality of nanowires form aliquid crystal phase in which each nanowire has a long axis orientedsubstantially normal to the first surface of the core.
 15. A waveguideas in claim 10, wherein the nanowires absorb light impinging on thefirst or second surface of the core and emit light, and wherein thenanowires are oriented within the core such that a majority of the lightemitted from the nanowires is emitted at an angle greater than thecritical angle crit, where crit=sin−1(nr/ni), nr is the index ofrefraction of the cladding, and ni is the index of refraction of thecore, thereby directing a majority of the emitted light toward at leastone edge of the core, providing a light concentrator.
 16. A waveguide asin claim 1, wherein the one or more nanowires or one or more branchednanowires comprise one or more of: a fluorescent material, asemiconducting material, a material comprising a first element selectedfrom group 2 of the periodic table and a second element selected fromgroup 16, a material comprising a first element selected from group 12and a second element selected from group 16, a material comprising afirst element selected from group 13 and a second element selected fromgroup 15, a material comprising a group 14 element, or an alloy or amixture thereof.
 17. A waveguide as in claim 16, wherein the one or morenanowires or one or more branched nanowires comprise one or more of:ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS,CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, GaN, GaP, GaAs, GaSb, InN,InP, InAs, InSb, Ge, Si, PbS, PbSe, PbTe, AlS, AlP, AlSb, or an alloy ora mixture thereof.
 18. A waveguide as in claim 1, wherein the one ormore nanowires or one or more branched nanowires comprise one or morematerials, each material having a band-gap energy between about 0.4 eVand about 4.1 eV.
 19. A waveguide as in claim 1, wherein the one or morenanowires or one or more branched nanowires are heterostructurescomprising at least two different materials.
 20. A waveguide as in claim19, wherein the at least two materials are distributed radially about along axis of the one or more nanowires or about a long axis of an arm ofthe one or more branched nanowires.
 21. A waveguide as in claim 1,wherein at least one collector which collects waveguided light isoperably connected to at least one edge of the core.
 22. A multilayerlight concentrator comprising: a stack comprising two or more waveguidesas described in claim
 1. 23. A multilayer light concentrator asdescribed in claim 22, wherein the two or more waveguides comprise oneor more nanowires or one or more branched nanowires that absorb light ofdifferent wavelengths, and wherein the waveguide comprising the one ormore nanowires or one or more branched nanowires that absorb theshortest wavelength light is located closest to a light source and thewaveguide comprising the one or more nanowires or one or more branchednanowires that absorb the longest wavelength light is located farthestfrom the light source.